It's not that there are suddenly packs of roving grandmas mugging people in the streets -- it's that we're living longer, so there are more elderly everything. In 2015, old people accounted for 10 percent of America's prison population, a 400 percent jump compared to 1993. As long as people continue aging and doing crimes, that type of growth is unsustainable. So what can we do about it? One idea is to transfer them to special geriatric units, but that would cost a lot of money in itself. Also, many of the old prisoners are against that, because they would lose their "informal caregivers" within the prison population. Like, maybe Skull-Kicker Pete saved Dicey Carlos from getting shanked 15 years ago, and now Carlos pre-chews his food for him.
Another approach is to release prisoners who are too old and frail to be a threat to society, but the process is such a bureaucratic nightmare that inmates keep dying off before they see freedom. One inmate, a drug dealer serving a life sentence, was passed up for the "compassionate release" program, despite being in a near-vegetative state. Then you have cases like the 83-year-old sex offender who was released to a nursing home because he was deemed "too sick to hurt anyone again," only for him to go and molest a 95-year-old woman and grope a hospital worker.