I know that in 4th grade - I was 9 or 10 - I was already having "naughty" thoughts about other boys, about being naked with them watching me and stuff, and not understanding at all the thrill I was getting out of it. Even as young as 7 or 8, my going-to-sleep make believe world would usually include me and my best friend at the time Brett getting married and living happily ever after. I never thought about my girl friends, only Brett, or occasionally another boy friend. (Even now, thinking back on it, it's hard for me to type "girl friend" without remembering the teasing I got about having girlfriends. Grade school boys are cruel!)
It took me until 7th grade (12ish) to realize what it meant about the naughty thoughts about other boys (and the other actions that had started going along with them). I repressed it, though, and in high school even went so far as to start calling one of my friends my girlfriend, until she caught me at it and gave me a tongue-lashing I'll never forget.
I didn't actually have sex, with anyone, until I met the guy who is now my husband, so I can't speak to that part of it. But yeah, I was definitely having the feelings, whether I admitted it or not, as far back as I have clear memories.
- dfp