My impression is that crime is concentrated among both young and male folks. It wouldn't surprise me that a disproportionate share of the prison population is young and male, especially in that do that increase. If it's due to non-violent crimes. One in 100 American adults is in prison or jail on any given day.
Becky Pettit of the University of Washington and author of Invisible Men talks with EconTalk host Russ Roberts about the growth of the prison population in the United States in recent decades. Pettit describes the magnitude of the increase particularly among demographic groups. She then discusses the implications of this increase for interpreting social statistics. Because the prison population isn't included in the main government surveys used by social scientists, data drawn from those surveys can be misleading as to what is actually happening among demographic groups, particularly the African-American population.