What Should We Do With Emerging Adults?

As neuroscience blurs the line between childhood and adulthood, courts are staking out a third lane 

On a September afternoon in 2011, two teenagers went out looking for trouble in the Dorchester neighborhood of Boston, Massachusetts. When Sheldon Mattis spotted two boys outside a convenience store, he handed a gun to his friend, Nyasani Watt, and told him to “go handle [it].” Watt rode toward the boys on his bicycle and shot them from behind, killing one and injuring the other.

Mattis and Watt were tried together, and the jury convicted both of first-degree murder; however, their sentences could not have been more different. Watt, the 17-year-old gunman, was processed as a juvenile and given the possibility of parole, while his co-defendant, 18-year-old Mattis, was tried as an adult and sentenced to spend the rest of his life in jail. 

The shooter was just 10 days shy of his 18th birthday on the day of the attack. Had it occurred a few weeks later, he too would have been condemned to die in prison. 

Free Will on Trial

Despite their disparate treatment by the legal system, the two young men were, neurologically speaking, nearly indistinguishable. Like all teens, their prefrontal cortexes—the cognitive hardware that helps humans assess risk, resist peer pressure, and consider long-term consequences—were not fully developed. Neuroscientist David Eagleman calls this region, which is the last part of the brain to mature, “the organ of socialization.” During this developmental middle ground known as “emerging adulthood,” humans’ higher executive function, moral judgement, and impulse control come online. 

Advances in neuroscience have forced courts to consider a fundamentally philosophical question: Is wrongdoing the result of a person’s free will or biological factors beyond their control? Eagleman skillfully framed the dilemma in a classic 2011 essay for The Atlantic, arguing that the question rests on a false dichotomy. According to Eagleman, biology and behavior are intertwined—our actions are shaped by our environment and experiences and cannot be disentangled from our underlying anatomy or genetics. 

This is not news to anyone whose business depends on assessing youthful risk. It is the reason that car rental companies and auto insurers treat anyone under 25 with an abundance of caution.

The fuzzy boundary between biology and agency bears directly on juvenile justice. We now have decades of research that back up what Supreme Court Justice Anthony Kennedy said “any parent already knows”: that the human brain does not finish maturing until age 25, after which people tend to grow out of delinquent behavior. If brain structure affects judgment and self-control, then courts cannot reasonably treat brain development as irrelevant. This does not dissolve accountability; instead, it demands a more utilitarian, forward-looking justice system. After establishing the facts of the case, the relevant questions are practical: How likely is this person to cause harm again? Is confinement necessary to protect the public? Which interventions can produce lawful behavior? 

In the case of Mattis and Watts, the two Boston teens given wildly divergent sentences for the same crime, science eventually had its day in court. In 2024, the Massachusetts Supreme Court ruled that life without parole for “emerging adults,”—defined as ages 18, 19, or 20 at the time of the crime—violated the state constitution. Chief Justice Kimberly Budd wrote that the scientific record shows this group has “the same core neurological characteristics as juveniles,” whose cognitive capacities are distinct from adults in consequential ways that must be considered at sentencing. As a result, Mattis and another 200 state prisoners sentenced to life as teenagers became eligible for parole after serving 15 years. 

A Third Lane

Although modern neuroscience holds explanatory power, we must remain aware of its limitations before moving every legal boundary to age 25 or transferring all emerging adults to juvenile court. A brain scan cannot tell a judge how blameworthy a particular defendant is, predict their future with certainty, or supply an empirically valid age of moral culpability. Emerging adulthood is a useful policy concept in applying justice, not an objective medical diagnosis.  

In the face of uncertainty, courts must exercise metaphysical and scientific humility. We have the capacity to retain a conventional adult track for serious crimes while building an emerging-adult lane that borrows from rehabilitative aspects of the juvenile system. 

Three distinct policy ideas follow from that approach:

• Offenses committed before age 21 should not trigger mandatory life without parole. 
  For long sentences imposed for conduct before 25, lawmakers should authorize a second look after a substantial term. California provides youth-offender parole hearings to most people convicted before age 26. Review does not guarantee release, but it allows courts to consider the individual’s original crime, their personal growth, and victim input to better assess future public safety risk.
  
• Lower-level cases should be eligible for diversion and probation tailored to emerging adults. Conditions must be concrete, comprehensible, and connected to the behavior at issue. Douglas County, Nebraska, offers judicially supervised problem-solving court to felony defendants ages 18 to 24. In Illinois, participants in Cook County’s Restorative Justice Community Courts for offenders ages 18 to 26 recidivate at 13 percent within a year of their crime versus 65 percent for young people processed in traditional court.
  
• Emerging adults should not be placed into the general adult prison population alongside hardened criminals.
  Specialized units for people ages 18 to 24 can improve safety inside institutions as well as prospects after release. A randomized controlled trial funded by the National Institute of Justice tested such a model built around credible messengers and individually tailored cognitive behavioral therapy and found that participants had 73 percent lower odds of committing another violent offense. 

Conclusion

Historically, the justice system has given us two choices: treat a person as a child, or treat them like an adult. While drawing a line at 18 has served a necessary legal purpose, it remains a poor demarcation of human development. Mattis was not necessarily more mature because his birthday came first, and Watt would not have crossed a magical threshold when he turned 18. Two options are not enough. Breaking out of this arbitrary duality requires adopting more granular, developmentally appropriate responses to youth crime. 

The best solutions will carry less metaphysical certainty about blame and more clear-eyed reason about what should happen next.