For tobacco, the interval for risk of starting use was similar, but the cumulative level was lower. There were no intercohort differences until around age 21 y, by which age around 40% in each cohort had used. Of notable significance, as for alcohol, were the changes across cohorts after this age. The highest overall level of use, reflecting continued initiation into the mid-20s, was in the second oldest cohort; lower cumulative levels (reflecting less initiation during the same period of life) were observed among those in the two youngest cohorts.
