At 18 years old, when I finally accepted I was a transsexual and needed to transition, and that no magic spell would save me. I started at 21 (minus 4 days), because of the delays that were put into the process for various reasons, by various people.
The special thing with me is that it's my own fault I couldn't transition early. I thought transsexuals were yucky old perverted ->-bleeped-<-s (thanks dad for the awesome opinions you've given me). A pedopsychiatrist who I was seeing for something else told me about transsexualism at the age of 11 or 12. She asked me if I wanted to be a girl, and I said no, not really, that I was just a boy who was more like a girl. She put a medical term on that and I was happy with that.
I was oh so convinced that magic or something would save me. Or maybe that conviction came a few years later, and the reason at that point was still that I didn't think I had the right to be a girl, because when I asked to be a girl in my childhood, I was told no. I'm not sure at all. My memory is awful. I don't even remember asking my mother to be a girl at all, actually; she told me a few months ago.
I had a chance to start therapy and possibly puberty blockers, something that the vast majority of transgenders have never come close to having, and I didn't seize it.
I don't know if one day, I'll be able to stop feeling guilty and full of regret for that action, and stupid for having always known I was not a guy yet taking 18 years to do the math and take a decision. It's so hard not to, not just for physical reasons, especially with how much brighter my life until now would likely have been if I could have even only been on puberty blockers and seen a therapist. It would have enabled me to build myself a normal adolescence, social skills and a social life.
My experience is why I think such things should be thoroughly investigated, so the shy/stupid/endoctrined/magical thinking/untrusting/whatever child effect can have less consequences. I know myself. I know if I had been thoroughly made to open up, to confess my feelings and beliefs, and if things had been explained to me properly, I would have changed my mind. But then again, I can't really blame anyone. I'm pretty sure most children would not have acted the way I did.