A few days before I am due to interview Jazz Jennings, the most high-profile transgender child on the planet, her publicist emails me a document entitled “Do’s and Don’ts”. It contains a list of questions I may and may not ask the activist and reality star, and a list of words I should and should not use, when speaking to Jennings in person or when writing about her afterwards.

The first thing I am instructed not to do (“do not” is written in bold block capitals) is say that Jennings was “born a boy”. Instead, it is considered “appropriate” to say either that Jennings was “assigned male at birth” or that people assumed she was male when she was born. This, in turn, means that Jennings should be “referred to with proper female pronouns”, even when describing her during the brief period before she started publicly identifying – “presenting” – herself as a girl, which she began to do at the age of five. Describing Jennings as “transgender”, as a “trans youth” or a “trans kid”, is fine. Using the collective noun “transgenders” is not. “Never say ‘tranny’ ,” the instructions warn. It is, they explain, “a slur”.

Jennings has long brown hair, a deep tan – she lives with her family in south Florida – and ever so slightly sticky-out ears. In conversation, she is articulate, which must partly be down to practice: she was first interviewed on US national television at the age of 7 and today, at 14, is regularly invited to speak publicly about her life as a transgender child. She has her own YouTube channel, advertises Clean & Clear face wash (“See The Real Me”, runs the campaign’s slogan) and, last year, was listed as one of Time’s “25 Most Influential Teens of 2014”. In other words, she is making herself visible, a fact she concedes is kind of ironic.

“If you know who I am as a person, you’ll know that when I get a lot of attention, I don’t really feel comfortable,” she says. “I’m not self-centred. The truth is that I’m willing to put my story out there for other people. I’m doing it to help you understand.”

This is a sentiment echoed by 16-year-old Cameron, a transgender teenager from Westchester County, New York, who is featured in Susan Kuklin’s book Beyond Magenta, a collection of interviews with trans kids. “I tell people, ‘I’m a boy, call me “he”.’ My birth name is irrelevant.” Cameron was a “girlie kid”, but that began to change as the years passed. It was at the age of 14 that Cameron came out as transgender to school friends. But it was actually Cameron’s father – a fireman – who mooted the idea of a gender reassignment. Cameron’s mother came along for the first hormone injections. “I’ve always been pretty decent about trying to explain things,” says Cameron. “My parents did their internet research and I tried to answer their questions in a non-alienating way. I tried to help them understand how much more comfortable hormones would make me feel.”

From Cameron’s perspective, stereotypical ideas about gender don’t make sense. “Males float around somewhere, females float around somewhere else, and some people don’t float at all – they swim. What I mean is, swimmers control where they are going. The swimmers do their gender instead of be their gender.”

Cameron and Jennings are just two of a new generation of teenagers who are willing to speak publicly about gender dysphoria, a condition previously known as gender identity disorder – a mismatch between biological sex and the gender a person identifies with emotionally and psychologically. A 2012 survey suggested 1 per cent of the UK population may have gender dysphoria, although, perhaps unsurprisingly, solid figures are hard to come by, due to the number of people who will never seek treatment for how they feel.

What we do know, however, is that we are witnessing a sharp rise in the number of British children receiving treatment for gender dysphoria. The Tavistock and Portland NHS Trust treats gender issues for those under the age of 18, and over the past six years, the number of children aged ten or under being referred to them has quadrupled. In almost 50 of these cases, the children were aged 5 or younger. Two of them were three years old. Against this backdrop, the parent of any young child could do worse than listening to Jennings and understanding who she is and how she became the girl she is today.

I am told not to inquire about surgery or her sexuality, although, as it turns out, she addresses both: she says that she has not yet undergone any form of surgical gender reassignment and that she likes boys. She admits, with a gentle stoicism, that the feeling is rarely mutual. “There are a lot of boys who think associating themselves with a transgender individual is gross,” she says. “They think if they date someone like me, then they’ll be called ‘gay’. I’m waiting to find someone who can push aside those judgments and accept me for who I am.”

Romance is just one of the challenges we see Jennings forced to negotiate in I Am Jazz, her new reality series. It is a remarkable piece of television, providing a window into the world of a teenager whose life is at once immediately familiar – she has friends round for sleepovers, she bickers with her older twin brothers – and yet almost incomprehensibly alien. Strangers threaten her with death online. She plays football for her school team, but only after her parents – Greg and Jeanette – fought a two-year legal battle to allow her to line up against girls rather than boys. She frets that the hormone pills she has been prescribed aren’t boosting her oestrogen levels enough, and that she won’t mature as quickly as her friends. It’s a complicated time – adolescence – somehow made about a million times more complicated.

Jennings was originally given the name Jaron, and began expressing the belief that she was in the wrong body when she was a toddler. In the first episode of I Am Jazz, her mother describes how a two-year-old Jennings asked, “When is the Good Fairy going to come with her magic wand and change my penis into a vagina?” She describes how she had nightmares in which beards and moustaches pursued her, threatening to attach themselves to her face. On her fifth birthday, she had a pool party and was permitted to wear a sparkly girl’s swimming costume for the first time. Soon after, her parents allowed her to present as a female, a process known as “social transitioning”. She chose the name “Jazz” after watching her older sister play Princess Jasmine in a school production of Aladdin.

“When I look back at photos of myself as a really young kid, I look into my eyes, and I just see a little girl,” she says today. “Maybe I was dressed as a boy, maybe I had short hair, but I was a little girl. I knew I was. People say, ‘How could you know?’ But they’re underestimating the intelligence of a two or three-year-old. At that age, you know the difference between genders. I liked Barbie dolls. I liked things that were pink and sparkly. That was just me. I knew what was going on in my brain.”

She emphasises that being transgender really has nothing to do with your anatomy. There are terms for individuals who have undergone sex reassignment surgery – “transexed”, for example – whereas to be transgender is simply to have the conviction that you are the opposite gender to the body you have been assigned. It’s a leap – partly intellectual, partly empathetic – she says many people still struggle to make, preferring to use the presence or absence of a penis as the gold-standard demarcation of gender. I get the feeling Jennings has had more people inquire about her genitalia than any 14-year-old should.

“People are always asking, ‘Did you get the surgery?’ And I always say, ‘It doesn’t matter what’s between my legs; it’s what’s between my ears that defines me,’ ” she says. It’s a line she came up with a while ago and one, she admits, grinning, that she is proud of. “People need to understand that your body doesn’t decide who you are. Your brain does.”

Which isn’t to say that Jennings does not want a female body. She does. When she was 11, she began taking hormone blockers and, some time later, oestrogen pills. The use of these hormone treatments is central to Jennings’ story, but it is also one of the most contentious issues surrounding transgender children and the question of how best to support them.

The hormone blockers themselves do exactly what it says on the tin – blocking either the testosterone or oestrogen in a child’s body and preventing them from sliding into a potentially traumatic puberty. “I definitely didn’t want to develop as a male, that’s for sure. That was my worst nightmare. To get an Adam’s apple, to get hairier, to get a deep voice,” says Jennings. “I think that would be any girl’s nightmare.”

The blockers are reversible. If you stop taking them, puberty kicks in. The rationale is that they buy a transgender adolescent time, so they can think about what they want to do. The oestrogen pills, however, prompt more permanent changes, such as the development of breasts and a higher-pitched voice. If you are a transgender boy, taking testosterone can help you develop certain male features, such as facial hair. From a parent’s perspective, the introduction of these “cross-sex hormones” is a leap of faith. Do you allow your 14 or 15-year-old son or daughter to embark on this process knowing there really isn’t any way back? If the answer is yes, then, upon reaching adulthood, your child may decide to undergo gender reassignment surgery. In the Netherlands, where this hormone treatment was pioneered, this programme is sometimes known as “12-16-18”, a reference to the ages at which each step is taken: blockers, hormones, then surgery, the results of which, in many transgender women, have been known to fool gynaecologists.

In Britain, the most controversial thing about this type of hormone treatment is not that it is widely available for transgender children, but rather that it is not. In America, recent years have seen the opening of dozens of clinics – many US states boast several – in which social workers, psychologists and endocrinologists work together to treat trans kids as they enter puberty. In the UK, we have the Tavistock and Portland NHS Trust. And that’s it. And while they have begun prescribing hormone blockers in some cases, this is as far as they will go. Cross-sex hormones are not made available. In other words, if Jazz Jennings lived in this country, she would not have been able to become the Jazz Jennings we see today.

Many British parents believe that their children could and should have benefited from the kind of treatment that is available in the Netherlands and United States. Susan Green is the mother of a transgender daughter, Sasha (not her real name), who is now in her early twenties. Green, who lives in Leeds, “always knew” that Sasha was a girl. “I had a two-and-a-half year-old who wanted to wear Pocahontas pyjamas and be the pink Power Ranger. Who was obsessed with the Little Mermaid and wanted Barbies for Christmas.”

Sasha was referred to the Tavistock, where gender dysphoria was confirmed. She had socially transitioned during her final year at primary school, and was happier than she had been in a long time. When she moved to secondary school, though, she was “absolutely annihilated”, according to Green. “She took seven overdoses in a year because of the bullying she got. This was at 11, 12 years old.”

It was at this point that Green inquired about the possibility of Sasha receiving hormone blockers, and was told it was not something the Tavistock would provide. The best they could do, according to Green, was offer to support Sasha through a full male puberty. “I told them, ‘No, you won’t,’ ” she says. “ ‘Because she’ll be dead. She will not live through that. It would stigmatise her for the rest of her life because every time she walked into a room, you would know [that she was biologically male].’ Sasha said she would rather be dead than feel as though her body was going against who she was on the inside.”

The rates of self-harm and suicide among transgender youth are shockingly high. According to research carried out by mental health charity PACE, 59 per cent of transgender young people in the UK have attempted self-harm, that figure dropping to just 9 per cent of all 16 to 24-year-olds. In the US, data published this year revealed that 45 per cent of transgender 18 to 24-year-olds have attempted to kill themselves. It is, according to Jennings, the main motivation behind her decision to document her life. “It’s one thing to change someone’s perspective on the topic of being transgender,” she says. “But if I can share my story and prevent one of those losses, then that is so much more.”

As for Sasha, Green felt her only option was to take her for treatment in America, to a clinic in Boston, where she received hormone blockers in time to prevent a male puberty. Later, the clinic provided her with oestrogen and, at 16, Sasha travelled to Thailand for gender reassignment surgery. Three years later, she was a Miss England finalist. “She’s beautiful,” says Green. “She’s happy.”

One result of her experience is that Green is now heavily involved with a support group for transgender children and their families. Called Mermaids, it is a network that is vocal in calling for hormone treatment to be made available in the UK. Deborah, a health and social-care lawyer from the southwest of England, is also a member of Mermaids. (A quick note on the name: it turns out that many transgender children, particularly girls, develop a strong connection with mermaids. Jennings has her own line of wearable silicone mermaid tails.) Her child was assigned female at birth and, despite being “a real tomboy” in childhood, it wasn’t until puberty hit that James – not his real name – became deeply unhappy.

“It was when he realised that his body was developing the wrong way – as female – that’s when it hit him,” says Deborah. “That there was something much more profound going on in terms of how he felt about his body.”

Much of James’s story echoes Sasha’s. He became depressed, he dropped out of education and made “a serious attempt on his own life” prior to being seen by the Tavistock. As James had already entered female puberty, it was considered too late for hormone blockers; all the NHS could offer was psychological support until he was 18 and old enough to be referred to adult services. Like Green, Deborah was not convinced her child would live that long, and so took James to see the same paediatric endocrinologist in Boston. By the time James was seen, he had “developed a bust”, something Deborah feels could have been avoided if he’d had access to hormone blockers sooner. He will have “top surgery” – the removal of his breasts – next year. In the meantime, James uses tight binders to hide them, but is otherwise much happier than he had been prior to treatment. “He just did a week in Cornwall, and wore a binder under a wetsuit while he was having surf lessons,” says Deborah, chuckling with disbelief.

The doctor who treated both Sasha and James is Dr Norman Spack, who in 2007 founded the first American clinic to treat transgender children. Spack is a warm-hearted, enthusiastic 71-year-old. He says he has treated “around a dozen” British children since opening his clinic, and a TED talk he gave in 2013 about his work has been viewed online more than one million times. He says his motivation to work with transgender children is a result of having treated many transgender adults and finding the experience “extremely painful”, because so many of them had to live with the trauma of never having the chance to be the person they felt they were. “By the time puberty begins, the child who says they are in the wrong body is almost certain to be transgender and unlikely to change those feelings,” he says. “Not doing anything for them not only puts all of them at risk of suicide, but it also says something about whether we are a truly inclusive society.”

But not everyone is so sure that giving trans children cross-sex hormones is the right thing to do. Dr Polly Carmichael is director of the Tavistock and Portman Gender Identity Service. “It’s complicated,” tends to be her unapologetic refrain when discussing her work. You get the feeling she is someone who has spent a lot of time patiently dealing with people who are outraged that her clinic is offering hormone blockers to 12-year-olds, and then a lot of time patiently dealing with parents who are outraged that her clinic is not offering cross-sex hormones to their transgender 15-year-olds. “There is not a ‘right’ and ‘wrong’,” she says. “There are no ‘goodies’ and ‘baddies’. There are just complex young people in the middle of all this who have a range of possible outcomes.”

When I ask why she won’t prescribe transgender teenagers under 16 cross-sex hormones, she says, “Because the NHS is an evidence-based service, and at this point in time there’s not enough evidence to support that.” In terms of treatment, what might seem like a no-brainer for individuals such as Jazz Jennings or Sasha Green – people who have had gender dysphoria all their lives – may not seem such a good idea when you’re faced with a teenager who has only just come to terms with the fact that they are transgender, but who desperately wants to do something about it. “Children are developing individuals. You have a young person who is exceedingly sure about their feelings and feels they definitely won’t change, and parents who understandably worry a lot about self-harm,” she says. “But someone can go from feeling that they want hormones and surgery to then deciding that they don’t.”

She also stresses that offering cross-sex hormone treatment as an off-the-shelf solution for trans kids doesn’t reflect what a complex and nuanced issue gender identity is. It’s not simply a question of being a girl trapped in a boy’s body or vice versa, which still represents a fairly traditional binary outlook on gender. Carmichael says that an increasing number of children and adolescents are placing themselves on a spectrum, using labels other than a trans binary identity to describe themselves, including “non-binary” and “gender queer”. These children don’t necessarily need hormone treatment so much as “the support to really explore their sense of self, their place in the world and who they want to be”. In other words, helping children to be happy with who they are doesn’t have to mean changing who they are.

“In the UK, we’re seeing much younger people socially transitioning,” says Carmichael. “But sometimes it then becomes almost impossible for them to think about the reality of their physical body. They are living totally the gender they feel they are, but of course their body doesn’t match that, and it becomes something that can’t be talked about or thought about. Clearly, it then becomes quite difficult in terms of keeping their options open and ensuring fully informed consent for any appropriate physical interventions.”

One thing everyone agrees on is that we, as a society, need to be a lot more understanding about the reality of life as a transgender child. Visibility is vital, which is why the existence of Jennings is such a big deal for many trans kids her own age. “I’ll see comments online from people who want me to die, or who want to kill all transgender people in general. When I see this, it just means I have to continue sharing my story until I am needed no more.”

In the meantime, we have to hope this slow shift towards normality continues. Deborah, the mother of James, describes how he and his 14-year-old brother lie on the sofa with their trousers rolled up, comparing leg hair. “When he began the hormone treatment, we said that this wasn’t going to make everything perfect. This is something that is going to help you be true to yourself and initiate some changes in your body. But sometimes life will still be crap. You’ll have exams to do and have rows with your friends, and that it wasn’t a panacea,” she says. “But living a lie and trying to be someone you’re not? That’s no life really, is it?”