The military has always discouraged, if not actually forbid, felons from being drafted or enlisted, although standards are historically relaxed when the armed forces are having trouble maintaining their recruitment goals. It is a sign that the military is consuming its resources faster than it can replace them.

The armed forces like to train their own killers, so they can be docile and amenable to discipline. The "Dirty Dozen" types are ready to kill, but not die, for their country. The same can be said for adults. In wartime, when anyone is liable to be drafted, the military even accepts men in their 30s or even 40s who cannot be indoctrinated as well as adolescents are. A grown-up can be trained how to fight effectively, but he is a lot less likely to risk his life than a teenager.

Unit loyalty and cohesion is easier to instill in young, socially integrated young men because it exploits their natural tendency to form gangs and cliques. (It's a primate thing.) When they start forming their own gangs
outside the chain of command, you've lost them.