I may be in the minority, but I personally didn't 'come out' to myself until 26 (a few weeks back). It isn't that I didn't know, it's that I was denying it/suppressing it and trying to just 'fit in'. I've always felt overtly feminine, and battled to be more masculine as a result. I've dealt with a lot of weird things in my life, family issues, health issues, religious issues, and for the first time in a long time (the last year or so) I have been the healthiest (physically and mentally) that I've ever been. This may be why it all suddenly came out, it was being shoved under the mental carpet because I had other things to deal with.
I wish I had come clean with myself when I was younger, so that I could have started to deal with it earlier, but I'm just glad I know now, since I finally understand my constant frustration and anxiety (and although it's still there, I at least understand it now, and can work on my GID).