It's been such a long time, and I don't remember a "definitive" moment in my life per se.
I do know that by third or fourth grade, I was regularly dressing up in my mother's clothes. (This would be in the 1970's.) I lived in a rural area, in a churchy environment, so I did not get a lot of exposure to the world in order to even hear about transsexualism. My mother would have been too naive to even really know about it, my father never really talked to us because he wasn't home much.
But by about fifth grade, I remember being so entranced by a book I saw on the shelf at the local library. It was called Conundrum (by Jan Morris), and I kept trying to sneak peeks at it. I remember being terrified that someone would see me, so I would take furtive glances, then kept putting it back. Obviously at that point in my life, although I don't remember what I was thinking, it's clear that the concept of transsexuality was something that I really connected with.
By sixth grade, I was miserable because I was not a girl. I was so envious of the girls -- their friendships, their clothes, their demeanor, I wanted badly to be part of their circle but couldn't be.
Throughout my teenage years, I knew about transition but had no idea of how I would do it... and to pursue it would disrupt my life tremendously and probably bring utter rejection by my entire family and social circle. So I never even really considered actually committing to doing it. I just dressed as much as I could when no one was home. I was also an escapist, with a wonderful imagination, and spent most of my time reading and being alone and imagining lots of different fantasies... so it was easiest at that time to "live in my head" and not live in real life. I never thought I would have to reconcile my outer world with my inner.
It was in my mid-20's where it finally hit me that (1) transition was possible and (2) I couldn't any longer live one life inside and a different life outside. So that's the first time when things got bad and I identified consciously as transsexual.
If I had been more aware of the cost I would pay for "suppressing" everything and thinking I could just live inside my whole life, if I had had a mentor who had transitioned or just someone who really understood everything, if I would have had a supportive home and social environment, if I hadn't been immersed in a conservative Christian subculture, perhaps I would and could have transitioned earlier. Pretty much my situation precluded any opportunity for an early transition.
At least now, I know what I am trading in. That's something, I suppose, and gives me assurance that I'm not missing something. I've tried life as a man and even during the good moments, things still were rather dreadful or at best "neutral" ... and never for long.