Individuals who reported ever using cannabis were queried about the age at which they had first used it (median age 16 years). Consistent with prior studies of early-onset cannabis use (25;26), use for the first time prior to age 17 was defined as adolescent cannabis use, which was reported by 27.1% of the full sample (55% of users). Twin pairs were considered discordant for adolescent cannabis use if one twin reported onset of cannabis use prior to age 17 while the other reported either no lifetime history of cannabis use or use at age 17 and older. Sensitivity analyses that (a) excluded lifetime non-users and (b) examined other age cutoffs for adolescent onset as well as continuous age were also conducted.
