The research arose from recognition that household survey data on age at first use of drugs can be biased due to sample selection and inaccurate recall. One potential concern is attrition, whereby individuals who become involved with substance use at an early age become increasingly less likely to be surveyed in successive years. Results of the present analysis suggested that attrition might have caused substantially less bias than did forward telescoping in the form of the inflating of age at first use over time. The indication of forward telescoping was particularly pronounced with respect to age at first use of alcohol. The analysis also presented a procedure for correcting the distribution for age at first use for forward telescoping but not attrition by considering a portion of the NHSDA data collected in successive years as constituting a cohort study. Results of applying this procedure with NHSDA data collected from 1982 to 1995 for participants born during 1968-73 suggested that prevention programs need to be introduced at an earlier age than would be indicated by uncorrected retrospective data. Tables, figure, and 24 references (Author abstract modified)