Research SummaryEmpirical evidence shows that no sharp change in cognitive functioning or in offending careers occurs on the 18th birthday. Many aspects of higher executive functioning, including impulse control, planning ahead, reasoning, thinking before acting, emotion regulation, delay of gratification, abstract thinking, and verbal memory, as well as resistance to peer influence, continue to mature through the mid‐20s. Most young offenders naturally “grow out” of offending in the early 20s, which is the peak period for desistance. Adult court processing makes offenders worse; convictions are followed by an increase in offending, juveniles who are dealt with in adult court are more likely to reoffend than other juveniles, and sending young people to adult prisons leads to an increase in recidivism. The evidence suggests that the rehabilitative approach of the juvenile court is more successful than the punitive approach of the adult criminal court.Policy ImplicationsMany justifications for the more rehabilitative treatment of juvenile offenders (immature, poor executive functioning, poor emotion regulation and self‐regulation, poor planning, more influenced by immediate gratification, low adjudicative competence, more susceptible to peer influence, more redeemable, less set in their offending habits, and less culpable) apply to young adult offenders. As in some other countries, there could be special legal provisions in the United States for young adult offenders 18–24 years of age. One possibility is that these offenders should not be dealt with in the adult criminal court but in special courts for young adult offenders that are more focused on rehabilitation. Alternatively, all young adult offenders could be assessed for their risks, needs, maturity, culpability, and adjudicative competence, and in appropriate cases, they should be given a “maturity discount.” Special efforts should be made to help young adult offenders with low intelligence and/or mental health problems. Young adult offenders should be housed in special correctional facilities and should be assisted with problems of reentry.