Evidence of increasing age of onset of cannabis use among younger Australians

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.