Here’s a graph you might find a bit interesting. Several thousand men on the internet were shown pictures of girls and women of all ages without being informed of their age and asked if they found them sexually desirable or not. Here’s the result:

http://omg.wthax.org/attractiveness_age.png

What’s interesting is how sharply the graph falls under 13. I imagine the men guessed the girls were under 13 and became reluctant to rate them as attractive. The true rating for girls under 13 is probably a bit higher than reported. But anyway, make of it what you will.