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Consider three young people: An 18-year-old who can vote, but can’t legally buy a beer; a 21-year-old who can drink, but is charged extra to rent a car; and a 25-year-old who can rent a car at the typical rate, but remains eligible for his parents’ health insurance.

Which one is an adult? All of them? None of them? Some of them? Or does it depend on the individual?

These questions are newly salient in the criminal justice system. Over the past year, several states—including Vermont, Illinois, New York, and Connecticut—have debated laws that would change how the justice system treats offenders in their late teens and early twenties. It remains the case that in 22 states, children of any age—even those under ten—can be prosecuted as adults for certain crimes. “Raise the Age” campaigns across the country are pushing for legal changes in order to treat all offenders under 18 as juveniles. But some advocates and policymakers are citing research to argue 18 is still too young, and that people up to the age of 25 remain less than fully grown up.

Some of the most compelling evidence comes via magnetic resonance imaging, or MRI. In 2011, brain researchers Catherine Lebel and Christian Beaulieu published a study of 103 people between the ages of five and 32, each of whom received multiple brain scans over the course of six years. The researchers were looking for changes in white brain matter, a material that supports impulse control and many other types of cognitive functioning. The majority of participants in the study, including those as old as 32, experienced increases in white matter connectivity between scans. In some parts of the brain, this connectivity increased by as much as 4 percent between the ages of 20 and 30, compared to as much as a 6 percent change between the ages of ten and 20. In a separate study of 403 children and adults, the same researchers and a group of collaborators found that the volume of white brain matter peaks around age 37. Altogether, the research suggests that brain maturation continues into one’s twenties, and even thirties.

“Everyone has always known that there are behavioral changes throughout the lifespan,” said Lebel, now an assistant professor of radiology at the University of Calgary. “It’s only with new imaging techniques over the last 15 years that we’ve been able to get at some of these more subtle changes.”

Researchers are using the term “post-adolescence” or “extended adolescence” to describe this period of development in one’s twenties and early thirties. Social change is as important as biological change in understanding why some people in this age group are drawn to crime. Individuals who are “disconnected”—neither working nor in school—are more likely to get in trouble with the law. While fewer young women are disconnected today than in previous decades, the opposite is true for young men.

In the 1960s, feminism and the advent of the birth control pill destigmatized premarital sex. Young women began to focus on education and career before motherhood or marriage, and couples were able to delay childrearing. Between 1960 and 2015, the average age at first marriage leapt from 20 to 27 for women and 23 to 29 for men. Fewer family responsibilities made it less imperative for young adults to work full time, and with the downsizing of the manufacturing sector, there were also fewer jobs available for young men without a college degree. Today, one in seven Americans between the ages of 16 and 24 are neither working nor enrolled in school. Among young black men, 26 percent fall into that disconnected group.

“Researchers have caught up with what parents have noticed,” said Terrie Moffitt, professor of psychology and neuroscience at Duke. “When my parents finished high school in the 1950s, you immediately got married, had your first child, and went into a job that you had for many years. Now we have the concept of a gradually emerging adulthood.”

Experts used to believe that “adult onset” criminals, or those who get in trouble for the first time in their twenties or older, were more likely than juvenile offenders to come from affluent backgrounds, and to have higher intelligence. New research questions those assumptions. For several decades, Moffitt has been studying a cohort of nearly 1,000 people born in New Zealand in 1972. In a paper published in March, she and a group of coauthors observed that the research subjects who were convicted of a crime for the first time as legal adults—at age 20 or over in New Zealand—had a lot in common with those convicted for the first time as legal juveniles. Both groups were likely to have had challenges such as low-income parents, behavioral problems dating back to childhood, and below-average IQs. Many of the common crimes committed by people in their twenties, such as driving under the influence, seemed related to impulse control problems typically associated with teenagers.

If people in their twenties are a lot like adolescents socially and biologically, should they really be considered full adults under the law? Many advocates who work directly with this population say no. “For many years, the idea of how to achieve public safety with this group was you want to lock them up, protect the community by not having them around,” said Yotam Zeira, director of external affairs for Roca, a Massachusetts organization that provides counseling, education, and job training to 17- to 24-year-old male offenders. “The sad reality is that after you lock them up, nothing gets better. Public safety is not really improved. Prosecutors know they are prosecuting, again and again, the same people.”

Zeira, the coauthor of a report on justice alternatives for this age group, sees three possible reforms: reclassifying young adults in their early twenties as juveniles, as is the case in Germany and the Netherlands; providing judges, attorneys, and probation programs more tools within the adult system to treat younger defendants with leniency and rehabilitation; or creating an entirely new young adult justice system “in between” the family and criminal court, with specially trained prosecutors and judges and less of a mandate to incarcerate.

Some progress has already been made. Nationwide, the incarceration rate of 18- to 24-year olds dropped by 28 percent between 2001 and 2013, according to a data analysis from the Program in Criminal Justice Policy and Management at the Harvard Kennedy School of Government. Some states want to push even further. In June, Vermont Governor Peter Shumlin signed a law that will allow some offenders up to the age of 22 to be referred to family court, where the focus is on rehabilitation, instead of criminal court, where the focus is on punishment.

In other states, the politics have proven more difficult. In February, Illinois State Representative Laura Fine, a Democrat, introduced two bills that would raise the age of adult criminal responsibility to 21, one just for misdemeanors, and one for both misdemeanors and felonies. According to Fine, prosecutors from the Cook County State’s Attorney’s office opposed even the misdemeanor-only plan, and she could not garner significant Republican support. “We weren’t optimistic it was going to go anywhere,” Fine said. “It’s going to be a matter of time and people getting used to the idea and learning more.”

The Cook County State’s Attorney’s office did not respond to requests for comment.

In 2007, Connecticut raised the age of adult criminal responsibility to 18, and Governor Dannel Malloy, a Democrat, has proposed raising it again, to 21, for all but the most violent felonies. That legislation has not moved forward, but the state is planning a new, separate prison to house 18- to 25-year-olds, in order to keep them segregated from supposedly more hardened, career criminals.

A similar compromise took place in New York. Last year, Democratic Governor Andrew Cuomo supported legislation that would have raised the age of adult criminal responsibility from 16 to 18 for most crimes. It also would have established specialized courts for young adult defendants and allowed some prisoners as old as 23 to remain in juvenile facilities. Republican legislators, prosecutors, and even some associations of defense attorneys opposed the plan, and it died. Governor Cuomo later issued an executive order establishing segregated prisons for teenagers convicted as adults.

While politically palatable, young adult prisons may not be all that successful in decreasing reoffending. Research shows that even detention in a juvenile facility is “criminogenic,” meaning it makes it more likely that a person will reoffend, compared to a juvenile who committed a similar crime, but was not incarcerated.

Beyond politics, one of the challenges of asserting that 18- to 25-year-olds are not full adults is that science shows some people in this age group are much more mature than others, with more static brains. “You can’t look at a brain scan from someone you don’t know and say that person is 18,” said Lebel, the brain researcher. “You can pick out any age, whether it’s five or 30, and you see people are distributed over a wide range.”

Moffitt, the psychologist, agrees that the policy implications of the new research are far from clear. “In our justice system, it has to be the same rule for everyone for it to be just and fair,” she said. “There will always be the sort of very serious, early onset kind of offenders that ... will have a crime career as a lifestyle.” There is also a “larger group of young people who are milling around, being young, getting in trouble, annoying everyone. But young people have always done that. You don’t want them to get a criminal record that prevents them from getting a job.”

The problem, Moffitt added, is that “as long as you make a cut point based on age, you are treating both groups the same.”