An alternative explanation for the differential mean age of first use could also be a cohort effect; adults have had more years to have actually used cannabis and this in turn results in their mean age of actual use being higher than that found among youth .. This sentence is hard to understand. A simpler version with equivalent meaning is the following:
The difference observed in the average age at which respondents started using cannabis can also be explained by a cohort trend . This is because adults have had more time to start using cannabis and , as a result , the average age at which they started is higher than that of young people .