The age cutoff is arbitrary. But when we do not treat it as inviolate, then we do us all a disservice. In practical terms, minors have no rights, and thus should have less responsibility. That is, they should never be tried as an adult, under any circumstances. It is always their parents' responsibility if their upbringing comes out wrong.
In the inner city, tons of young kids get adult sentences for being involved in the drug trade. So, it is already far from an inviolate already. There is a disproportionate portion of young people convicted as adults who are racial minorities, so that's another big issue with the legal system at present. But, it's silly to say that not being mature enough to automatically be trusted to sign your own contracts inherently means that you should be excused for rape.