Criminal gangs specifically recruit legal minors because they are able to do things legal adults can't: Repeatedly and with impunity violate the law. Whether it's smuggling or assassinations, using legal minors make it all a lot less risky.
Would it stop stupid kids from doing stupid kid stuff? No. Would it make minors less attractive to organized crime? Yes.
No. For both children and adults, sentencing strictness only deters a SINGLE category of crime--white collar.
Most other forms of crime, especially violent, are almost completely insensitive to the harshness of any possible punishment sentence.
If a brain is sufficiently broken that it no longer has the limiter against harming another human being, prison sentences won't do anything to fix that.
Wikipedia article makes no obvious mention of correlation between sentencing and deterrence. Linked article, in fact, demonstrates that alternative programs provided almost all the improvement.
