Well, all throughout childhood I was pretty boyish. I knew that I didn't fit in with the female groups at school, so I hung out with the boys, played rough sports, got in fights throughout my teenage life and took much pride in fighting like a man. As I got older I didn't really think much about it, I just thought what everyone else thought; I'm just a tomboy. I kinda came to the realization that something was kind of off about me though when I first started using the internet, since literally the first thing I ever did on the internet at the age of 12 was go into a chat room, and lie about my sex. I acted the way I normally acted, I just didn't like being seen as a female, and the internet seemed like the perfect place to be seen the way I wanted to, since people couldn't really see me, and no one took a second thought about it. I did that for years and years (still do). I knew about transsexuals, but I didn't really put much thought much about how it concerned me, I just lived my life. It wasn't until recently that I've started thinking about why I act the way I act, why I have such low self esteem and why I really don't like my body and it just kinda hit me....I would say it was a pretty slow hit though. First I just kinda came to the realization that there's more FtM's than I thought, then everything just slowly started clicking. I was in a bit of denial at first, but then I started thinking about how boyish I always acted since childhood, how much penis envy I had growing up, how I always related to males and always wanted to just blend into a male crowd without sticking out, how I always enjoyed being viewed as a man on the internet more than anything, how I always wished I would be viewed as a man in the real world.
I guess it took about two years to come fully to terms with it, so I would say I didn't fully know until I was about 20.