Categories of ‘early’ and ‘late’ age at first use were created by dividing distributions of age at initiation into two approximately equal groups. For alcohol, early first use was defined as 16 years or younger, late as 17 years or older (using the lower of the two possible cut-offs to create amore conservative definition of early initiation). For cigarettes, early first use was defined as 14 years or younger, late as 15 years or older. Definitions of early vs. late age at first use of cannabis were the same as those for alcohol: 16 years or younger for early and 17 years or older for late. As described in Section 2.5.2, never-users were also included in the models.
