WHEN I was growing up, there was a saying, ‘Sixteen will get you 20,’” remembers Eloise Anderson, director of California’s Department of Social Services.  “’Sixteen” is a girl’s age; “20” is the number of years an adult male would spend in prison if he had sex with her. For years, statutory-rape laws languished in disuse. But recent studies show that a significant proportion of teens are impregnated by adult men, prompting politicians to once again apply such laws, which remain on the books in all 50 states. At the federal level, last year’s welfare-reform law called on states and localities to “aggressively enforce statutory-rape laws.”