While arrest rates for juvenile offenses have substantially decreased since the 1990s, U.S. national trends in conduct problems are unknown. Population variation in conduct problems would imply changes in the social environment, including emergent or receding risk factors. The present study separated age, period, and cohort effects in conduct problems using nationally representative surveys of 375,879 U.S. students, conducted annually, 1991-2015. Summed score of seven items measuring frequency of conduct problems was the outcome. Conduct problems have decreased over the past 25 years among boys; the total amount of the decrease was approximately 0.4 standard deviations (p < 0.01), and by item prevalence, the total amount of the decrease was 8-11%. Declines are best explained by period effects beginning ~2008, and a declining cohort effect beginning among those born after 1992, suggesting not only population level declines, but more rapid declines among younger cohorts of boys. Trends were also consistent with age-period-cohort effects in evenings spent out, suggesting one possible mechanism. Conduct problems among girls were lower than boys and did not demonstrate trends over time. These changes may reflect the changing nature of adolescence towards less unsupervised interaction.