I would say about 11/12. As a child, I always knew I was different to the other boys, but obviously without all the hormones and whatnot I was yet to feel an actual attraction to boys.
At the start of high school I used to date girls, I guess because it was the "norm" and in my head, I thought I must be attracted to them if I can date them.
Then I remember this really, really insanely hot guy in our year and I used to go all red and get butterfly's whenever I saw him, that was probably when I realized. I remember thinking that he was making me feel, how a girl "should" make me feel.