In three large, nationally representative surveys of U.S. 12th graders, college students, and adults ('N' = 9 million) conducted 1968–2015, Americans became significantly more supportive of legal marijuana (cannabis) starting in the mid-1980’s. Hierarchical models using age-period-cohort analysis on the adult (General Social Survey) sample showed that the increased support for legalization is primarily a time period effect rather than generational or age effect; thus, Americans of all ages became more supportive of legal marijuana. Among 12th graders, support for marijuana legalization was closely linked to perceptions of marijuana safety.