The interval of risk for starting cannabis use began in late adolescence and continued far longer in life for all cohorts, but very large intercohort differences were observed here, with the oldest cohort dramatically lower than all others, and the two youngest cohorts much higher than the second oldest cohort. Clear and important age-specific differences were also evident across cohorts: the two youngest cohorts were very similar until the age of around 18 y in their cumulative incidence of drug use—but initiation of cannabis use continued to occur at a higher level following this age. A similar and more marked difference between the second and third cohorts was also evident, with initiation of cannabis use highly unlikely to occur for the second oldest cohort after the age of around 25 y. Clearly, then, two trends are evident: early onset use is greater for the three youngest cohorts than for the oldest, but much more marked is the fact that with each successive cohort, there is a prolongation of the period of risk for initiation of cannabis use far beyond adolescence.
