The number of prisoners over the age of 55 serving more than one year in state prisons increased from 26,300 to 131,500 in the last two decades, according to a study released this week by the Department of Justice, Bureau of Justice Statistics.

Research has shown that all but a small minority of criminals, even violent ones, mature out of crime before middle age, meaning that long sentences for aging offenders does little to prevent crime.

Homicide rates peak at age 19, according to The Marshall Project, while arrest rates for forcible rape peak at 18. Some crimes, such as vandalism, crest even earlier, at age 16, while arrest rates for forgery, fraud and embezzlement peak in the early 20s.

The BJS study, entitled “Aging of the State Prison Population, 1993-2013,” written by E. Ann Carson and William J. Sabo, found prison admissions for people 55 and older increased by 82 percent between 1993 and 2013.

The average sentence length for prisoners older than 55 was 82 months in 2014, higher than the 69 months for the 18- to 39-year-old prisoners, and the 71 months for the 40- to 54-year-old prisoners.

Research by Carnegie Mellon University Professor Alfred Blumstein has found that for the eight serious crimes closely tracked by the FBI — murder, rape, robbery, aggravated assault, burglary, larceny-theft, arson and car theft — a span of five to 10 years in the life of an offender is the typical duration during which these crimes are committed.

Property criminals, like burglars and car thieves, tend to stop in the 20s, while violent criminals are more likely to continue into their early 30s. Drug-crime careers can be lengthier, yet long sentences have had little effect on drug crime. “When you lock up a rapist, you take his rapes off the street. When you lock up a drug seller, you recruit a replacement,” Blumstein told The Marshall Project.

Why does society lock away those least likely to commit crime for the longest periods of time?

To start with, state and federal sentence guidelines have not kept up with evolving science and research. Most states, and the federal government, have guidelines for assisting the court in sentencing. Although, not bound by those guidelines, most judges fall in line.

Sentence guidelines are normally based on two factors — the seriousness of the offense and the defendant’s criminal history. The more serious the offense, combined with a long history of crime, the longer the sentence.

It is difficult for a 19-year-old to accumulate a long criminal record. However, a 40-year-old has had 22 years to collect criminal convictions.

There are actuarial tools — risk assessments — available that can predict the dangerousness of an offender. These tools target younger offenders who are more likely to offend — and more likely to be violent — than their older counterparts, yet the guidelines call for longer sentences for the aging offender.

Many court systems use risk assessments to enhance decisions regarding parole release, probation supervision and, increasingly, to determine pretrial detention and bail decisions.

Pennsylvania is about to take a step most states have resisted for adult defendants. According to Fivethrityeight.com, Pennsylvania is about to bring risk assessment into the sentencing equation.

The Pennsylvania Commission on Sentencing has been charged with incorporating a risk assessment tool into the state’s Sentencing Guidelines. When implemented the new guidelines could allow those considered low-risk offenders to get shorter prison sentences and those deemed high risk to spend more time in prison.

The result would be to keep those young violent offenders we fear behind bars and those aging offenders, we’re merely tired of and mad at, out of prison and off the public dole.