A 35-year follow-up study based on a nation-wide population study of the life expectancy of people with intellectual disability (ID) was undertaken. The study population consisted of a total of 60,969 person-years. A prospective cohort study with mortality follow-up for 35 years was used and the life expectancy of people with ID was calculated for different levels of intelligence. Proportional hazard models were used to assess the influence of level of intelligence and associated disorders on survival. People with mild ID did not have poorer life expectancy than the general population and subjects with mild ID did not have lower life expectancy in the first 3 decades of life. In cases with profound ID, the proportion of expected life lost was > 20% for almost all age groups. The female preponderance was manifested from the age of 60 years onwards, 25 years later than in the general population. Respectively, survival between sexes differed less. Epilepsy and/or hearing impairment increased the relative risk of death for all levels of ID. The prevalence of people with ID over 40 years was 0.4%. People with ID now live longer than previously expected, and the ageing of people with mild ID appears to be equal to that of the general population, posing new challenges to health care professionals.