good thing, I don't see us being at a place yet where we can help someone who is that damagedHe would certainly be evaluated repeatedly whilst in juvenile detention. By the way, he can be found insane whilst still a juvenile and committed. The two processes are separate, and the 21 age line does not apply to the mental health system.
Juvenile detention tries to focus on rehabilitation much more than the adult penal system. That includes much more psychological/psychiatric work.