To put it in crude terminology that some have used in my area: Edit:Bleep There is no species other than humans on the planet who refrain from sexual activity after reaching puberty, and humans do it only because of societal pressure. In most of the history of civilization, an 18-year-old was considered middle-aged. Marie Antoinette was married at the age of 14, proceeded to have 4 children, and was executed at 37. In even earlier times, that age of 37 would have marked her as a very old woman.
Young women today tend to go out of their way to appear older than they really are. A lady of my acquaintance is very self-assured and independent. She drinks responsibly, smokes (although I've tried to talk her out of it), and has been living with her boyfriend for 2 years. She just turned 17. If I ran across her in a bar, without knowing her previously, I'd try to nail her without ever considering that she might be a minor.
By the same token, the producers of the "Mini Pops" records and videos are under investigation in the UK for possibly promoting pedophilia. I can see why; those kids can't be more than about 12, but they're dressing, acting, and singing just like pop stars such as Britney Speers. I suspect that they wouldn't go to that extent without adult coaching.
All that parents can do is educate (preferably with the help of the school system) their kids about sex before it actually becomes a part of their lives. You can't stop them from doing it, other than by locking them up, so just make sure that they know how to do it safely and responsibly.