http://www.juvenilejusticefyi.com/hi...e_justice.html
this seems a somewhat unbiased summary of how juvenile laws changed over time. it seems some changes intended to "protect" kids may have actually moved them closer to being exposed to adult sentences.
originally (early 1800s), if you were over 7, you went to jail- there weren't really 2 systems. then a juvenile system was set up, that was mostly all rehabilitation and no punishment (or at least intended to be). then in the 60s the Sup Ct. ruled that juveniles were entitled to all rights afforded adults in the criminal justice system. 1 dissenter (Fortas) wondered if the Court wasn't forcing the juvenile defendants closer to the adult system.
After that paranoid states legislators enact laws that let kids be tried as adults. I guess the prosecutors are saying some kid's actions are so bad that the reasons for the separate system don't apply because the kid ain't going to possibly be "fixed."