I think it was early in my final year of primary school (12yo, I don't know the equivalent grade in US education system) when I first started having thoughts about boys or girls sexually. I remember sneaking in my friend's porn (VHS!!!) and had my first sexual encounter at 13 with another boy who bunked in the dorm next door (yay for all-boys boarding school!).
I agree with MikeL, kids will think of sex at any age, and it'll probably happen faster if he/she has friends who have had experience. Peer pressure and all that stuff. I don't think it's wrong, but parents could try to prevent them having sex early, and just think about it until later on when they're a bit older. But in my culture, parents very rarely or don't give their kids "the talk" at all, so we're basically left to fend for ourselves. We were expected to stay chaste until marriage. Sucks, but hey, what they don't know wont hurt 'em