Because age demographics of prison are very distant from age demographics of people committing crimes at the time they commit them for a variety of reasons, starting with the fact that imprisonment happens after conviction which happens after the crime. But also because of both discretionary leniency to first time offenders (which younger criminals are more likely to be) and younger criminals even apart from whether they are first time offenders, and because of formal enhancements to punishment for repeat offenders, which combined mean that, for otherwise similar immediate predicate offenses, older criminals are more likely to be imprisoned at all and likely to be imprisoned longer.