Age at First Use was queried if the respondent reported having consumed at least a full alcoholic drink (alcohol), trying a cigarette (cigarette) or using marijuana (cannabis). Use/age at first use was broken into 4 categories for each substance: never, early (lowest quartile of age at first use distribution), average (middle 50%), and late (highest quartile). (See Table 2 for substance-specific age ranges.) As there is no standard definition of early initiation of substance use, we chose the lowest quartile of the age distribution to represent early use relative to same sex peers in the same birth cohort.
