Laurence Steinberg isn't someone I agree with often, but he, Thomas Grisso, Elizabeth Scott, and Richard Bonnie have this op-ed in the NYT opposing the crackpot notion of raising the juvenile court jurisdiction age to 21.
The proposal to expand the jurisdiction of the juvenile system to age 21, in addition to being based on ambiguous science, would also create two potentially serious policy problems. First, just as the adult correctional system is ill equipped to respond to the needs of adolescents, the juvenile justice system is poorly positioned to handle young adults. It is hard to imagine a juvenile facility that could appropriately house 20-year-olds and 14-year-olds, or a juvenile justice staff whose training would allow it to work effectively with young adults. And because a disproportionate number of serious violent crimes are committed by individuals between 17 and 21, the juvenile system would be overwhelmed by the number of young adults it would need to process, and its rehabilitative purpose could be seriously undermined.
Second, the juvenile justice system interacts with several other health and child welfare systems. Those agencies have created relatively separate systems for serving children and adults, in part because of important differences between these two ages. For example, some mental illnesses arise only in young adulthood, and professionals have long specialized in providing services either to children and adolescents or to adults. Creating a juvenile justice system that works well for both adolescents and young adults would require significant (and costly) restructuring of many other agencies.