Physical fitness is as important to aging adults with ID as in the general population, but to date, the physical fitness levels of this group are unknown. Comfortable walking speed, muscle strength (grip strength), muscle endurance (30 s Chair stand) and cardiorespiratory endurance (10 m incremental shuttle walking test) were tested in a sample of 1050 older adults with ID, and results were compared with reference values from the general population. Across all age ranges, approximately two-third of the entire study population scored "below average" or "impaired." Even the youngest age groups (50-59 or 50-54 years) in this sample achieve similar or worse results than age groups 20-30 years older in the general population. Low physical fitness levels in older adults with ID demonstrate that this group is prone to unnecessary premature loss of functioning and health problems, and maintaining physical fitness should have priority in practice and policy. (Contains 4 figures and 3 tables.)