Timing of first alcohol use was defined as age at consumption of first full standard alcoholic drink. Ages at onset were structured as a categorical variable of 0 = onset prior to age 16, 1 = onset age 16–17, and 2 = onset subsequent to age 17. These age cutoffs resulted in roughly equivalently sized groups for White (but not Black) twins and satisfied assumptions of multivariate normality (p > .05).
