An annual, national survey of high school seniors, Monitoring the Future, is the source of the self report data on over 3,000 respondents per year from 1975-1985. The analysis of age trends covers ages 17 through 23. Both self-report and arrest measures reveal substantial declines in illegal behavior throughout this age span; the one exception was arrests for assault. Rates of assault were shown to increase from 1975 to 1985, a previously unreported finding. For other offenses, time trends were found to be erratic. 8 tables, 1 figure, 34 references, 1 appendix. (Author abstract modified)