Nature vs. Nurture, Part Infinity.

Now Scientific American weighs in on the question with a piece, "Do Genes Time One's Loss of Virginity? (my emphasis):

...a new study suggests that the genes that drive puberty also influence some of the next stages of sexuality: age at first intercourse and—for women—age at first birth.

Aha! I knew it. But:

Of course, genes are not the only factor. Parenting, religion, social mores, peers and many other factors come into play. But researchers at the University of Cambridge estimate that genetics can explain about a quarter of the difference in the likelihood that an individual will have sex relatively early or wait to start.

Actually, I'm of a mind right now in which I give at least some of the credit (and blame) for all of our behavior to environmental factors.

But, remember, Nature vs. Nurture -- either way it's your parents fault.