Munchkin, you are right about cervical cancer being more of a risk for women who were sexually active early in life. It's because the organs are developing rapidly up until 17 or 18 and any trauma on them (like sex) can cause mild dysplasia. Mild dysplasia unmonitored can develop into cancer (hence why doctors will scare the everlasting hell out of you if you ever return an abnormal Pap test). Not to mention that I'm sure these 12 and 13 year olds are not going to the gyno.
I at most wanted to make out with a boy at that age. Not that I did. Kind of shy at that age.
Sex was something much older people did. My mom gave me tons of books to read when I was 6 or 7 that explained things from a biological and social perspective, and I really think real information (not playground talk) is better than keeping your kids in the dark. They're going to be curious and they're going to find out anyways, so it's not wise for parents to pretend sex doesn't exist.