Under 18...somebody's pregnant? Get the District Attorney on the case....that's a possible case of statutory rape..(depending on the state ((some are 16)). Time to start prosecuting for statutory rape. If the boy is 16 or under...prosecute the girl as well.
I don't know if that's true in the majority of cases, though I'm not a sociologist. I also agree that threats of violence don't necessarily happen in the majority of cases either. But I'd bet what does happen in a large percentage of cases is that the "boyfriend" (if you can really call him that) puts tremendous pressure on the girl however he can. And thanks to these laws taking away the parental influence, his influence is the only one she has.