Carly at Pornblography quotes Earnest Greene:

Thirteen is the new eighteen, and has been for a decade. By 18, most young adults are already sexually active and have been for some time. Better nutrition and pediatric medicine have been steadily lowering the age of first menstruation among American girls to the current record-breaking average of age 11. It’s just preposterous to expect young people to remain sexually inactive for SEVEN YEARS after the onset of puberty.

Preposterous indeed, as I noted some time ago in a discussion about the merits of porn.