Before I started transitioning, I already had a therapist, who I was seeing for other reasons. I got to trust her, but when I brought myself to talk to her about trans stuff (when I was in very early stages of admitting to myself), her reaction was something like, do you need to put a gender on yourself? Can you just be you? At the time I was still pretty confused so I didn't really stand up for myself much. I tried to bury it for a while and put my homemade binder away, but then it was back within a couple of weeks. So I arranged to see a gender therapist.
I do think I could have explained myself better to my first therapist, but it was a big deal and I always go blank when it comes to stuff like that. She helped me a lot, helped me get over abuse and I would like to go back to her just as a general therapist one day - but it would have to be as a man and I'd make it very clear.
Anyway, to answer your question I'm 26 and I just sort of blurted it out to my shrink a few months ago.