I realized I was different when I was like 7 or 8. I didn't know what gay was and I definitely didn't realize I was paying extra attention to guys, but a difference was apparent to me and it certainly was apparent to my mother.
I fully understood my sexuality when I was in fifth grade though (so 11?). Like, I had to go to a week-long camp with my fifth grade class and omg bunking with a bunch of guys in a small ass cabin...