A minor is a legal fiction it can and will be discarded when convenient. Many people either don't understand or pretend not to understand that order is the only thing preventing so called children from killing and eating you.
I can't remember seeing countries with stronger protections for minors collapsing into a perfect spherical body of criminal children. Good juvenile justice systems are not about the absence of consequences, they usually are about age-appropriate consequences instead. In short: supervision, intervention, and rehabilitation instead of pretending a child is simply a small adult. The idea that only the threat of adult punishment prevents children from “killing and eating you” is so simplistic it is an insult to everybody forced to read it.
If a kid becomes violent usually you have a long history of publicly visible and known smaller, less dramatic events leading up to that. Good social work and educational systems are able to pull children and teenagers out of that spiral, by for example isolating them from destructive family dynamics. All tried and tested, papers written about it, statistics and studies published about the effectiveness.
Countries having problems with criminal minors are usually countries who try to not spend any money on social measures and education. Or put guns into the hands of kids. There aren't many such countries.
Would it stop stupid kids from doing stupid kid stuff? No. Would it make minors less attractive to organized crime? Yes.
But that doesn't much matter - because deterring other children is not a major motivation for the "sentence 'em as adults!" crowd.
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.
https://en.wikipedia.org/wiki/Crime_in_El_Salvador
Turns out it works.
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.