What can we do to shrink our prison population, the world’s largest?

Most answers to that question point forward: They look to reduce future arrests, prosecutions and sentences. But such changes, while desperately needed, do nothing for the hundreds of thousands of people who are already serving long sentences in America’s expensive and overcrowded prison system.

And make no mistake about it: There are a lot of people serving extraordinarily long sentences. The state prison population grew 222 percent from 1980 to 2010; the National Research Council attributes half of that growth to an increase in incarceration time. The Sentencing Project reports that one in seven American prisoners is serving either a life sentence or its functional equivalent. (In some states, the number is almost one in three.) Once, parole boards could truncate some of these long sentences, but the decimation of parole has largely eliminated that possibility.

The explosion in sentence length has turned some prison wings into de facto nursing homes, with prisons responsible for providing costly medical care to a growing elderly population. Keeping people locked up for so long does little for public safety. Most people who commit crimes, including violent crimes, do so while young. Arrest rates for violent crimes peak during people’s late teens (rates for robbery, for example, are highest at age 19), and criminal careers for violent crime typically last only five to 10 years.

Middle-aged and older prisoners are an especially good bet for early release, particularly when they receive support during the re-entry process. For example, in 2012 a Maryland court ordered the release of nearly 200 prisoners who had served sentences of more than 30 years, mostly for homicide and rape. Fewer than 1 percent have committed a crime in the years since release.