Not that I want to play sociologist here, but there is a more basic force at work in these lives.
Without solid parenting, these kids hit puberty without any built in restraint on their urges. I'm not speaking only of their sexual urges, but any urge.
For guidance, the 12-13 year olds look up to the 17-18 year olds who are smoking, drinking, screwing, fighting and drug dealing. That's the adult life they see. They go home, and Mama is off at the club - drinking, smoking, screwing and fighting - while grandmom looks after them.
That's the life. Run by bullies and dedicated to getting high and getting laid. Not much emphasis on education, not that math or literature will benefit them in getting high or getting laid, so why bother?
Their lives are run by one simple dictum - If It Feels Good, Do it!
We need to make the consequences of "doing it" be more painful than the pleasure of "doing it", and the consequences of not doing what is expected of them more painful than the effort it takes to do them.
