Aim To determine whether declines in the prevalence of cannabis use in Australia have been accompanied by changes in age of onset of cannabis use.
Design A retrospective cohort study. To account for right censoring error we contrasted the mean age of onset for comparable age groups across the four surveys conducted from 1998 to 2007. Kaplan–Meier failure graphs were used to describe how the cumulative risk of first use of cannabis varied across birth cohorts born from 1947 to 1993.
Setting Australian data collected in the nationally representative, triennial, National Drug Strategy Household Surveys (NDSHS)
Participants A total of 88 268 Australian household residents aged 14 years and older.
Measurements Life-time use of cannabis and age of first use.
Findings For respondents under the age of 20 years, mean age of first use of cannabis has increased from 14.6 years in 1998 to 15.2 years in 2007.
Conclusions The decline in cannabis use prevalence that has occurred since 1998 in Australia has been accompanied by an increase in age of first use among those aged under 20 years.