It's hard to say that a certain event was indicative of my realisation of who I am, especially in childhood. Looking back now, it's easy to see the signs, but saying I knew what was happening at the time isn't right. Probably up until the age of 14, I guess, I knew what I felt, but not why. And before that it wasn't so much about gender, but more a general 'wrongness', that I attributed to everything under the sun. When stuff started happening to my body that didn't fit with how I thought it should be, I blocked it out. And I mean totally. Oddly I didn't think "this is wrong" or "why isn't it like that rather than this?". I kinda just... went into a sort of mental shutdown.
I don't think I actually allowed myself to dwell on it too much, and things being what they were, I wasn't really allowed to anyway. This led to me being pretty introverted and very... hmm... I guess shy is the closest word, as a kid. I figured everyone just hated me, probably because I hated myself, or... people had learned how to act a certain way and I just hadn't, or... I just had one of those personalities that 'didn't fit in'. Part of it was probably that I went through those years on a sort of autopilot, I guess a defense mechanism to protect my mind from everything happening that, had I allowed myself to think about it, would have led to things being far worse.
Spent a lot of time on my own, even at school. But that's something which is best left forgotten. However, a by-product of this is that I never really socialised with either boys or girls there, I was left to my own devices because the school had a policy of punishing the victims of bullying rather than the perpetrators and I spent the best part of 3 years isolated from everyone else at the back of a classroom with my work brought to me by a teacher because it was "for my own good". Naturally I blamed myself and figured I just wasn't cut out to be around people. I was a 'thing', some sort of nondescript mass. I would watch the different classes in the room socialising though, the interactions... but in an oddly distanced way. I guess this 'observer mentality' has stayed with me throughout my life.
Anyway, long story short, I guess when I actually realised who I was, was at the same time I allowed myself to accept who I am, and not find every other possible rationalisation imaginable for the way I felt. That came... hmm... I guess in my early 20's.