Keith Humphreys is a psychiatry professor at Stanford University.

The U.S. imprisonment rate has been shrinking for six years, but the change has been uneven across generations. Despite criminal behavior typically peaking in young adulthood, the young rather than the old are driving the nation's ongoing de-incarceration.

Over the most recent decade of state prison data analyzed by the Bureau of Justice Statistics, the inmate population aged even faster than the graying U.S. general population. The imprisonment rate for people ages 55 and older bucked the broader de-incarceration trend by jumping a startling 71 percent.

Adults younger than 30 in contrast were far less likely to be imprisoned in 2013 than was the case a decade ago. If the entire population had experienced the same change, states would be shuttering empty prisons coast to coast. This good news about young American adults is paralleled in other studies showing that they are far less likely to get arrested than were young adults of prior generations.

Multiple factors account for the rising proportion of older Americans in prison. First, ever the trendsetters, baby boomers are somewhat more criminally active in late life than were previous generations. Second, the many state-level reforms designed to reduce incarceration were implemented long after the “tough on crime” era in which many older inmates were given protracted sentences. Third, older convicted criminals by definition have had more time than younger ones to accrue long criminal records, which often leads judges to mete out longer sentences for a particular offense.

Because prisons are legally responsible for providing health care to inmates, the aging of the prison population could strain their budgets despite the decline in the imprisonment rate. On the other hand, elderly, severely ill inmates pose minimal risk to public safety and thus are often good candidates for compassionate early release.

The millennial experience of crime and criminal justice could not be more different from that of their parents and grandparents. Millennials grew up in an era of collapsing crime rates and as young adults are experiencing dramatically decreasing rates of arrest and imprisonment. If their experience is replicated in their children’s generation, the United States has a real chance of returning to the low-crime, low-incarceration environment it had for much of the 20th century.