I knew very early, like 6 years old. I always wanted boys to chase me on the playground, or to "fight over me", LOL. I had so many little boyfriends back then, it's funny to think back about it now.
When we played house, I always wanted to be the "mom". When we played School, I always wanted to be the teacher.
I probably vocalized that I was gay to myself when I was about 11 or 12, I think, when I actually had a physical attraction to other boys, and acted on those impulses. I never really tried doing the straight thing... I never had any girlfriends, although plenty of friends that were girls. It WAS uncomfortable, because I had a lot of girls attracted to me, and I never wanted to hurt their feelings.
My mother knew, of course, but we didn't really talk about it until I was 21 or so. I was brought up in a very liberal household, so it was basically a non-issue. My Dad and I never had anything resembling a positive relationship, so I've never really cared what he thought. Well, that's not altogether true. There was a time when I cared, but I've gotten over it. 
It always intrigues me when I meet someone who only realized "later in life" that they were gay. I always wonder if they'd always had impulses which they just suppressed... or if they legitimately had a "change of heart", for some reason. Most of my friends all knew when they were young, like me. The few people that I've known who had the later-in-life realization, were typically the ones that "fought it", and weren't comfortable with the idea of being gay.
