On the bus back from the cinema, a conversation drifted over from the back row. A mother questioning, curious, her speech accented; her teenage daughter, with perfect RP, fielding her inquiries with the exasperated patience that flourishes between the ages of 13 and 21.

“No, Mum, you’re a cis woman because you’re the gender you were born as.”

“OK. And what about Lily?”

Lily – or, perhaps, Daisy or Rose – was a school friend who was now using the pronoun “they”. The heavy overtone of the daughter’s forbearance was that these were matters her mother could not understand.

Among internet-literate teenagers, gender has become the primary way to challenge the mores of older generations. I know four journalists – London-based, middle class – whose children have announced that they do not consider themselves to be girls. It seems too many to be a coincidence. And if pained teenagers are now explaining gender fluidity to their mums on the 108 from Millennium Leisure Park West, you know the idea has truly gone mainstream.

We should welcome young people challenging gender, an arbitrary system that has acquired the status of immutable human nature. Name almost anything now associated with women – high heels, long hair, the colour pink – and you can find a time or place when it was considered masculine. And just as feminists once fought for “Ms” alongside “Miss” and “Mrs”, people should be allowed to take gender out of their honorific altogether and go by “Mx”. Getting used to “they” as a singular pronoun is harder but not impossible. Language evolves.

However, there is more to the current Gender Revolution than upending our assumptions about the “correct” names or pronouns or hobbies or appearance for men and women. In the past few years, the word “transsexual” has dropped out of favour – it is considered impolite to reference sex – in favour of “transgender”. But this obscures the idea that to cross definitively from one gender to another requires surgery and a lifetime of synthetic hormones. For trans men, it’s top surgery – breast removal – and, more rarely, a phalloplasty to make a penis, plus testosterone (“T”), which lowers the voice, hardens fat to muscle and unleashes any latent male-pattern baldness. For trans women, oestrogen (HRT, used off-label) can be supplemented with breast implants and a procedure to skin the penis and invert it, creating a neovagina and clitoris.

These surgeries are non-trivial – I have a friend undergoing the latter this summer and she will be housebound for two weeks afterwards, with a 12-week recovery period. Infection is always a risk. For her, it’s a life-saving intervention: she says she simply would not want to live in a male body.

But 80 per cent of gender-nonconforming children do not grow up to be transsexual; many emerge as happy gay men or lesbians content to live in their birth sex. A strange taboo has sprung up about mentioning this, as if the way that some people do not turn out to be trans invalidates the experiences of those who do. It should not.

But separating dissatisfaction with the social constraints of gender from body dysphoria is vital. Because we have smudged together the categories of “transsexual” and “transgender”, is every youngster who questions their gender – and, frankly, every youngster should, because gender is restrictive bollocks – getting the message that they must bind their breasts or tuck their penis? I wince when I read oh-so-liberal parents explaining that they knew their toddler son was a girl when he wore pink and played with Barbies. Is there really anything so wrong with being a boy who wants to dress up as Elsa from Frozen? Or a girl who would rather be outside getting muddy than wear skirts and be “ladylike”? Toys and children’s clothes are becoming more gendered: when I was young, we played with Lego – not “Lego” and “Lego for Girls”. As we have shrunk the boxes, is it any wonder that more and more children want to escape from them?

In the year to March 2015, the Tavistock in London – the only specialist gender clinic in the country for under-16s – saw 697 children. This year, it saw 1,419. The largest surge has been among girls aged 14 and over and it is this group I feel most personal affinity for, because, if I were growing up today, I would be among them. A few years ago, I found a textbook from my junior school, with three sentences that floored me: “My name is Helen. I am nine years old. I am skinny.” And the truth was, I was skinny. I had a bowl haircut and wore culottes. Then puberty hit and I piled on a few stone in a year. Taut pink skin turned to lumpen fat and mottled flesh. And everyone had an opinion about it. I was trapped inside a body that didn’t feel like mine any more.

Many of my school friends felt the same way. Some tried to escape through vomiting or starving. Others were part of that charmed cohort who became lissom, beautiful, golden; their parents felt a different sort of ­worry and they were treated to sermons about getting into strange men’s cars.

I won my body back by defacing it; at least, that’s how my parents saw it. An earring, then two. And another. Then piercings that no one could see: nursing each one like a wound or a child. Salvation through pain: a metal bar through cartilage that couldn’t be slept on for a month. A tattoo that hurt like hell. Pink hair, ebbing to orange in a shower that looked like Carrie. And finally – finally – a body that felt like me.

I tell my story not to belittle anyone else’s, or to imply that they have chosen the wrong path. If you cannot live in your body, then change it – and the world must help you to do that. But if you feel crushed by society’s expectations, it might be that there’s nothing wrong with you. There’s something wrong with the world.