As a group, people with intellectual disabilities (ID, formerly mental retardation) have a shorter life expectancy than the general population. However, there is considerable variation. It really depends on the underlying cause of the disability. But, there is good news. The life expectancy for people with ID has dramatically improved over the past few decades. Twenty years ago, people with Down syndrome were not expected to live past age 35. Today, these same people would live to 55 years of age. The average life expectancy for people with ID is in the sixties. The average life expectancy for people without intellectual disabilities averages in the seventies. Some people with intellectual disabilities are at greater risk than others. It depends on the cause of their disability. For instance, people with Tay-Sachs do not usually survive childhood. Individuals with Cockayne syndrome Type I are expected to die in their teens. People with Cockayne type II usually die before the age of seven. Cardiovascular disease and diabetes are common complications in Williams syndrome. The complications cause premature death. Individuals with Down syndrome are prone to Alzheimer's disease. Once diagnosed, they usually die within eight years of diagnosis. These specific disorders are discussed here.
Because there are many causes of intellectual disabilities (ID, formerly mental retardation), there is a wide range of severity. Severity ratings refer to the degree of functional limitations. Some people with ID are only mildly impaired. They can function independently with a just few additional supports. Other people's disabilities are more severe. They require a more diverse and intensive support structure in order to function adequately. Severity is discussed more detail in the section on Diagnosis.