Healthy development is a fundamental right of the individual, regardless of race, ethnicity, or social class. Youth require special protections of their rights, in part owing to vulnerabilities related to psychological and brain immaturity. These rights include not only protection against harm but opportunities for building the cognitive, emotional, and social skills necessary for becoming a contributing member of society. They apply to all youth, including those within the adult criminal justice system, which raises the legal question of when adult capacity and responsibility begin and special protections are no longer warranted. This article highlights (a) empirical findings from developmental science on when psychological and neurobiological development reaches maturity; (b) the extent to which this scientific knowledge guides current policies and practices in the treatment of youth in the United States; and (c) emerging policies in the treatment of young people in the justice system based on developmental science.