> the single most reliable way to reform a criminal is to let them get older without going to jail

I think it’s just getting older. If they’re a recivifism risk, locking them up until they’re older can be argued to serve the public good. If they’re not, you’re right, don’t jail them. The argument is essentially against short prison sentences; either find another way to punish or commit to locking them up until they’re 60.