Introduction: Persons with mental retardation (MR), particularly individuals with Down syndrome (DS), have been found to have attenuated peak exercise responses in previous research. However, it is unknown if the pattern of age-associated changes in HR or aerobic capacity (V? O2peak) is similar to individuals without disabilities. Purpose: To investigate age-related changes in aerobic capacity in persons with MR, with and without DS, using a retrospective analysis. Methods: Data were collected in university research laboratories, using 180 persons with MR without DS, 133 persons with MR with DS, and 322 persons without disabilities in four age categories: 9-15, 16-21, 22-29, and 30-45 yr. Subjects underwent treadmill testing with peak HR and V? O 2peak measurements. Results: Relative V̇O 2peak was lowest for persons with DS across all age groups. V̇O 2peak did not change after 16 yr in the individuals with DS, whereas the other groups exhibited a slight decline (∼10 mL·kg -1·min -1) with age. Peak HR was overall different between all three groups (P < 0.001), and the youngest age group had the highest peak HR versus the other three age groups (P < 0.0001). Conclusions: Persons with MR without DS exhibit similar age-related changes in V̇O 2peak and peak HR as individuals without disabilities. Similar age-related changes were also observed for peak HR in individuals with DS, despite overall lower levels. However, ?relative V̇;O did not decline with age in persons with DS, suggesting that aerobic capacity exhibits a different age-related response in individuals with DS.