Abstract Background We present a formal age–period–cohort analysis to examine if the recent increase in past-year marijuana use among the young is specific to the younger generation or if, instead, it is part of a general increase present across cohorts of all ages. This is the first age–period–cohort analysis of past-year marijuana use that includes adult trends from 2001 to 09. Methods Data come from the National Survey on Drug Use and Health, a series of annual, nationally representative, cross-sectional surveys of the U.S. civilian, non-institutionalized population. The analysis focuses on the 25 year time span from 1985 to 2009 and uses the recently developed ‘intrinsic estimator’ algorithm to estimate independent effects of age, period, and cohort. Results The recent increase in past-year marijuana use is not unique to the youngest birth cohorts. An independent, positive influence of cohort membership on past-year marijuana use, net of historical period and age effects, is smaller for today's youngest cohorts than it was for the cohorts that came immediately before, and, in fact, is at its lowest level in three decades. The recent increase in marijuana use among the young is more consistent with a historical period effect that has acted across all cohorts. Period and cohort trends differ substantially for Hispanics. Conclusions The major forces that drive trends in past-year marijuana use are moving away from cohort-specific factors and toward broad-based influences that affect cohorts of all ages. Strategic public health and policy efforts aimed at addressing the recent increase in past-year marijuana use should do the same.