Although the population with intellectual disabilities (ID) is increasingly growing older, there seems to be an early onset of functional decline in this group, which could be explained by frailty. We used data from the Healthy Aging and Intellectual Disability study (HA-ID) to measure frailty in people with ID. Frailty was measured with an adapted version of the frailty index, consisting of 50 health and age related deficits. We were the first to measure frailty with a frailty index in this population, and therefore its validity, in terms of predictive value, needed to be established. In the current article we provide an overview of the design of the frailty index and its relation with adverse health outcomes. In a nearly representative study population of 982 50-plus older adults with ID, we studied the prevalence of frailty and its validity over a 3-year follow-up period. Results show that people with ID were earlier and more severely frail than people from the general population. Frailty was related to early mortality, to disabilities in daily functioning and mobility, to increased medication use, and increased care intensity, but not to hospitalization. Using a hypothetical model, we identify possible interventions to increase the healthy life years in people with ID.