The flip-side of an aging society with declining fertility is that older people, with fewer children are likely to be less sympathetic to children, and you could see the incarceration rates increase, or remain steady, as less severe infractions are punished more harshly.
We recently saw this play out in the Queensland, Australia, state election where the opposition party, which was pretty much out of ideas, ran a scare campaign about youth crime in regional areas. Neighbourhood Facebook Groups where CCTV footage of "suspicious youth" are a mainstay and an aging population did the rest of the job and they won the election and passed "adult time for adult crime" laws: whether you agree with these or not, "adult time" in Australia means that the youth incarcerated will be adults in their 20s and 30s when they get out.
The Australian state of New South Wales routinely strip-searches young children, but again, there isn't much outcry.
It will be interesting to see how this plays out elsewhere. The worst case scenario is that kids will be politically scapegoated ("why should childless and aging taxpayers fund education?"), and it leads to a further decline in fertility rates.