Goals This study examined: (a) crystallized/fluid intelligence trajectories of adolescents and adults with Down syndrome; and (b) the contribution of endogenous (health, activities of daily living—ADL) and exogenous (cognitively stimulating leisure activities) factors on adults’ intelligence with age. Method Four cohorts (N = 80) with Down syndrome participated: adolescents (ages 16–21) and adults (ages 30–45, 46–60 and 61+). All completed Vocabulary and Similarities (crystallized) and Block Design and Raven (fluid) intelligence tests (WAIS‐IIIHEB, Wechsler, 2001). Results The 30–45 cohort significantly outperformed the 16–21 cohort. Except for Vocabulary, which remained stable, onset of decline was at 40–50. Age‐related declining health and ADL correlated with participants’ lower fluid intelligence, but cognitive leisure activities mitigated this influence. Conclusions Intelligence development into adulthood supported the continuous trajectory and compensation age theory, rather than accelerated or stable trajectories. Not only endogenous factors but also exogenous factors determined intelligence levels in adults with Down syndrome, supporting cognitive activity theory.