I'm in two minds about it - yes, it will possibly prevent unwanted pregnancy, but then saying that, what 13 year old is going to go into a store and actually buy the pill? Many would probably need to get the money off their parents, and I can't imagine they'd want to have that conversation with their parents, so in a way, although in theory it would prevent pregnancy, what is the likelihood that a significant number of underage girls will actually get the pill? Also, if the girls do get the pill, they might either not know about STD's, or they might feel pressured from the guy to have sex without a condom and so there's a risk of STD's spreading more.