Children officially become adults at age 18, or do they? The U.S. has struggled to answer this question. Kids can drive at 16, they can join the military at 18, but they can't drink alcohol until they're 21. Or in Colorado, use marijuana until that age as well. Apryl Alexander has been studying this question of when children become adults, particularly in the area of criminal justice. She's a psychology professor at the University of Denver. She spoke with CPR's Andrea Dukakis.