The obvious answer is 'as young as possible, and certainly before the onset of puberty'.
Yes...but when I look back on how I was at age ten or so, it seems utterly unreal that the person I was then could have articulated to her parents how she felt inside, and what she wanted her parents to do about it.
The actual me at ten was still deeply immersed in childhood, and completely naive and uninformed. It was 1962. Families were led by the Man Of The House; in my case a benevolent figure; but my Dad would not have understood anything about gender dysphoria. He was 42, and had learned many things in the war. Like his entire generation, the war had broadened his mind, and called his attention to many new things. So he could take a view on the characters and goings-on in D. H. Lawrence's Lady Chatterley's Lover, even if it was still regarded as a scandalous novel, unfit for the eyes of women and children. Army life - his eventual rank as sergeant-major - must have shown him all kinds of human behaviour. He must, for instance, have become well aware what a homosexual was, and what 'they' were supposed to get up to, even if the subject of same-sex love was taboo in public discussion. I suspect that my Mum did not know exactly what 'they' did, and Dad certainly wasn't going to tell her.
In this climate, anything about sex or gender would get mixed up and horribly complicated by misinformation and the lurid prancings of 'camp' entertainers, as seen on TV and in theatres everywhere. I had to endure the squeam-inducing performances of Danny La Rue, Kenneth Williams, Dick Emery, Lionel Blair and many others - and even Liberace - who played the cross-gender or homosexual card in various ways. Most comedians used jokes about sexual or gender variance as part of their stock-in-trade; although made quite inexplicit, so as not to cause embarrassment to the audience, who knew full well (or thought they knew) what was being referred to, but didn't want it thrust in their face because it was all 'dirty'.
It did not help that my Mum seemed to enjoy many of these acts, and found them wonderfully entertaining, and the costumes gorgeous. Danny La Rue was especially confusing and embarrassing to me. He wore a wig, dresses and high heels, and make-up that although ultra-feminine did not disguise the fact that he was really a man. He hadn't had surgery. Nor did he ape a woman's voice in a convincing way. I suppose he had to make it obvious that it was all just an act. If he had tried too hard, people would stop enjoying what he did and said, and he'd suddenly be a freak, and pilloried or reviled in a Sunday newspaper.
Throughout the 1960s, the dreadful phrase 'sex change' was associated - in the UK at least - with perverted sexual practices and louche immorality, something unmentionable in normal conversation, or at best a sensational and daring story about the wicked ways of film folk in The People or The News of the World. (I can see that the World is going to remain a metaphor for vulgar and salacious journalism for years to come) No compassionate insight or understanding - or treatment - was available for people who felt uncomfortable with their bodies.
It was therefore, in 1962, impossible for any child to talk about the subject to their parents. Children were not in any way associated with the 'disgusting' world of sexual deviance, and any attempt to open a conversation about wanting to be a little girl instead of a little boy would have got a shocked and possibly frosty reception. I would have been slapped and sent to my bedroom as a precocious little horror. Certainly not referred rapidly to a London specialist, and put on appropriate medication until I was older.
Even if I'd had extraordinary parents with amazingly advanced views, I couldn't have found the words. I could not have reduced my vague feelings of discomfort and distress to a definite concept, with a definite name that my parents could look up and read about, and then discuss with the family doctor. I couldn't say: 'Dad! I think I've got gender dysphoria! Look - it's all here on this web page!'. (Funny how I'd probably have approached my father first, assuming that I dared to speak at all; Mum was inclined to be opinionated, not a sympathetic listener, and wouldn't have been the one I'd want to break the news to) In 1962, gender dysphoria didn't exist as a public concept in the UK, and there was no Internet, no Wikipedia, no trans sites, no trans blogs. There wouldn't be for another forty-odd years.
So I'm saying that for me, and for everyone of my generation, an attempt to begin transition in 1962, when I was ten, would have led to ridicule or punishment or both; and in any case no diagnosis could have been made, and no treatment would have been available at the time.
It would have been the wrong time to transition.
And that would have remained true well into adulthood, although the 'punishment' would have been on the lines of social ostracism, a job lost, or promotion denied. It's only now that being trans is entering into the public consciousness as a condition that requires sympathetic and urgent treatment. And 'entering' is almost putting it too strongly. Trans people have a toehold on a mountain whose summit is still far off. But at least the climb has begun, and it is getting media attention of the right sort.
I visited Jane Fae at the Brighton Nuffield Hospital on Friday and again yesterday. She is still being video'd by the people who are making a documentary about her and two other transitioners for ITV3. It will be screened in the autumn. (Jane describes in her own words how her post-op experience is going on her blog: see my blog list) I met the producers of the documentary. From how they seemed to be, and what they got on camera, I think the programme will be sensitive and insightful, and a 'must-see'.
I just hope I didn't say anything silly when chatting in Jane's room with her family. There's a possibility that I will show up on the odd clip or two. I had to sign a consent form, just in case I don't get entirely deleted. You have been warned.