When I was 6 or 7 I asked my mom if there was "an operation that could make me a boy." She laughed and said no and it wasn't talked about after that.
I cut my hair when I was 13. I had always worn boys' clothes since I was a kid. I thought about hormones starting at around age 18 when an ex of mine began his transition but I was adamantly against it for a while, I think as sort of a defense mechanism, and now I really regret not starting T back then.
At age 22 I told my therapist that I wanted to take hormones, she was supportive and she called the local LGBT health center and talked to the person who deals with TG issues. Interestingly, this person told my therapist that I did not "fit the description" of a transsexual because I did not absolutely loathe my body. Thinking back, that really ticks me off!
When I was 24 I got a new therapist who specialized in TG issues. Several months into seeing her I told her that I wanted to start T and she wrote me a letter. I began T in June 2008 at age 25. I'm now 27.
Like I said, if I could do it all over again I would definitely have started T much younger, but I was afraid. For one, my hips didn't fully stop growing until I was 22 or 23 so if I had been on T that would have likely been prevented.