The U.S. Supreme Court has issued a series of landmark decisions regarding the culpability of juveniles under the age of eighteen and has increasingly referenced developmental science in these opinions. Still, the line between juvenile court jurisdiction and criminal court jurisdiction varies widely among state laws, as do the minimum ages for other legal or regulatory purposes. Although the operative age of “adulthood” typically falls somewhere between the ages of eighteen and twenty-one, it has been set lower in some important policy contexts, such as the age at which an adolescent is subject to criminal prosecution and punishment. Legal distinctions between juveniles and adults have been based on changing political climates and conventional wisdom rather than empirical evidence. Policymakers have drawn these lines without fully examining or understanding the developmental characteristics of these individuals and how similar they are to younger or older individuals in their behavior and judgment. Scientific evidence of human brain maturation shows continued development into the early twenties. In this Article, we summarize recent behavioral and neural findings on cognitive capacity in young adults (eighteen to twenty-one) and highlight several ways in which they bear on legal policies relating to the “age of adulthood.”

Richard J. Bonnie et al., When Does a Juvenile Become an Adult? Implications for Law and Policy, 88 Temple Law Review 769–788 (2016).