The 20th-century shift to punitive policies toward offenders in these two age groups is first reviewed. An increase in the number of homicides committed by adolescents and young adults in the late 1980s resulted in all States passing laws to make their juvenile justice systems more punitive, and these new laws led to more juveniles being tried and sentenced to adult prisons. These changes may have been counter-productive, however, as recent studies have shown that juvenile justice system services and supervision are more effective than confinement in reducing antisocial behavior. Criminal justice policy for adolescents and young adults must recognize what research has determined regarding child and adolescent brain development, i.e., that there are developmental differences in the human brain, which is not fully mature until early adulthood. Evidence from research on the effectiveness of criminal justice policies and the findings of developmental neuroscience require criminal justice policy reforms. First, consider raising the minimum age for criminal court to 21 or 24. Second, consider creating special correctional facilities for young adult offenders, with tailored services such as cognitive-behavioral therapy, drug treatment, mentoring, educational and vocational training, and work release programs. Third, consider a “youth discount” for young offenders that decreases the severity of penalties. Fourth, conduct risk and needs assessments of young offenders to guide intervention. Fifth, use reentry services that include therapy, drug treatment, and educational programs.