I'll throw my two cents' worth here...
Based SOLELY on personal experience, I'd say that gender identity starts very early in life. As far back as I can remember, I knew that I was supposed to have been born a girl. Back then, T* was pretty much unheard of, and I bought into the idea that I was somehow broken (or worse). The conflict between who I am inside and who I'm *supposed* to be has always been my own private hell.
I can only hope that some day soon, people learn to accept that some kids are truly T* and that this doesn't make them 'bad' or 'wrong'.
If a child wants to do more than to play dress up once in a while, then good parents should take steps to be sure to do what's best for their youngster's mental/emotional/physical well being. NOT what others and their agendas think is best - but what really IS.
I truly wish I'd had that option when I was young. I would have leaped at it!!