Respondents were queried about whether they had ever used alcohol, tobacco cigarettes (i.e. nicotine) or cannabis. Those who had used each substance were further asked to recall the age at which they had first used the substance. For all three substances, the median age of onset was 16 years. For analyses (i.e. model 1) in which SI or SA were the dependent variables, early substance use was defined as onset of each substance prior to age 15, based on the bottom quartile of the distribution of ages of onset. Supplemental Figure S1A – S1C show the distribution of ages of onset for each substance.
