Any cross-sectional survey research has limitations [46]. Some of the observed cohort differences might be traced to higher mortality among individuals in the older cohorts who began drug use at an early age. Nonetheless, we believe that differential mortality is unlikely to explain the rather large differences in cumulative incidence for illegal drug use across age groups observed: in the case of cannabis, with substantial age-related variations observed, there is no convincing evidence of substantial premature mortality [47]. Conversely, the evidence of tobacco-related premature mortality is substantial, but tobacco use showed the least prominent age-associated variation.
