In the past decade, developmental brain research has had an important influence on juvenile crime regulation. More recently, advocates and some policy makers have argued that the developmental research should shape the law’s response to young adult offenders. Developmental scientists have found that biological and psychological development continues into the early 20, and that 18 to 21 year old adults are more like younger adolescents than older adults in their impulsivity under some conditions. Further, like teenagers, young adults engage in risky behavior, such as drinking, smoking, unsafe sex, using drugs, and offending, to a greater extent than older adults. The possibility that much risky behavior, including involvement in criminal activity, is a product of psychological and social immaturity, raises the question of whether the presumption of reduced culpability and greater potential for reform should be applied to young adult offenders as well as to juveniles. This essay explores the potential implications for justice policy of the recent research on young adults. In doing so, we emphasize the importance of not exaggerating the available research findings, which do not indicate that individuals between 18 and 21 are indistinguishable from younger adolescents in attributes relevant to offending and punishment. Thus, we are skeptical on both scientific and pragmatic grounds about the merits of the proposal that juvenile court jurisdiction should be categorically extended to age 21. But the research does suggest that young adults, like juveniles, are more prone to risk-taking and that they act more impulsively than older adults in ways that likely influence their criminal conduct. Moreover, correctional reform is justified because young adult offenders, like non-criminal young adults and juvenile offenders, are more likely to become productive members of society if they are provided with the tools to do so during a critical developmental period. Policymakers today can draw lessons from the developmental model that has shaped juvenile justice reform. At the heart of this reform is a conception of adolescence as a distinct stage between childhood and adulthood. Juvenile justice programs increasingly respond to the developmental needs of adolescent offenders, as the best means of reducing crime and promoting their productive engagement in society. The developmental stage of young adulthood has taken on heightened importance as a period of preparation for adult roles. We conclude that the research supports a regime that recognizes young adults as a transitional category between juveniles and older adult offenders.