This study found very strong cohort differences in illegal drug use in particular. Although this may reflect actual cohort differences in drug use, they may also reflect response biases. Retrospective reporting of age of first drug use may be subject to error, given that respondents are being asked about events that, for older persons, may have occurred decades ago. Longitudinal studies of adolescents have found that estimates of the age of first use do tend to increase upon repeat assessment (i.e. as people age), but the rank ordering for different drugs remains the same [48–50].
