I for one was an innocent virgin when I married. I had no idea what sex was and even believed sleeping with a boy (actually sleeping in the same room not even in the same bed) would caused a girl to become pregnant. I know with TV movies magazines billboards the web etc. sex is so in your face today unlike just a few years ago when I was growing up.
I have friends and family who has young daughters. I know they are boy crazy way too young but even so I don't believe any of them are interested in sex on any level. They just want to hold hands maybe as far as kiss a boy.
I honestly believe you need to keep your attention turned towards young ladies of legal age. Don't worry about how young a girl might be when she starts to become interested in more adult matters such as sex. I don't condone sex with underage partners nor will I encourage it on any level even by speculating on how young a girl might begin thinking about boys in an adult way.