My moment was more like about a couple weeks.
I had always known what I wanted to be, and I semi-told my friend when I was 5-7, I completely told my mum when I was 7-10, I told other people when I was 13, but when I was that young I didn't know anything about transition or anything. When I became a teenager at some point I learnt about the whole transgender thing, but by that time I had so internalized my feelings as part of a wide range of attempts to fit in, the idea of me doing something so unusual seemed out of the question, additionally I didn't really know anything about hormones or the concept that someone could actually fit in and be perceived as female if they weren't born that way. I had this mindset of 'well, ok, in my mind I'm female, but I'm trapped in this male experience and there's nothing I can do so I'll just go with it. So I ignored the problem entirely.
Then when I was 19 I found out through randomly stumbling on a news article that it really was possible, what happened between that and realizing that I actually WAS one of these transgender people and that I COULD have done something about it YEARS ago, and that I was SO CLOSE in the past, I don't exactly remember. It took a few weeks, maybe a month, to truly come to realize it.
So yeah, no lightbulb moment, just stumbling across what I knew but didn't accept (i was transgender), and that it was actually possible to do something to correct the situation.