The present study examined age differences in performance on the Tower of London, a measure of strategic planning, in a diverse sample of 890 individuals between the ages of 10 and 30. Although mature performance was attained by age 17 on relatively easy problems, performance on the hardest problems showed improvements into the early 20s. Furthermore, whereas age-related performance gains by children and adolescents (ages 10-17) on the hardest problems were partially mediated by maturational improvements in both working memory and impulse control, improved performance in adulthood (ages 18+) was fully mediated by late gains in impulse control. Findings support an emerging picture of late adolescence as a time of continuing improvement in planned, goal-directed behavior.