Intellectual disability (ID formerly mental retardation) is not a specific disease. It is a specific disability with many causes. Therefore, there is no specific time of onset for intellectual disability. However, diagnostic criteria require childhood onset, before the age of 18. If the problems begin after age eighteen, the correct diagnosis is neurocognitive disorder. The symptoms may be identical to an intellectual disability. The difference is whether someone has lost a prior level of functioning. In this case, it is called a neurocognitive disorder (formerly dementia). If someone never developed this level of functioning in the first place, it is called an intellectual disability. It is possible to have both diagnoses. For instance, suppose a young man with an ID, suffers a head injury at age 21. If this injury caused a loss of prior functioning, then he would have both and ID and neurocognitive disorder.
An estimated seven to eight million Americans have an intellectual disability (ID). This means about one in ten families in the United States are affected by an ID. A 2010 study estimated that 5 million children in Europe have an intellectual disability and Australia reports that about 3% of their population has an intellectual disability and that it is the most common primary disability reported in that country.
Previously, the American Psychiatric Association's Diagnostic and Statistical Manual (DSM-IV, APA, 2000), reported 2-3% of people meet the criteria for ID. This was because the diagnostic criterion required an IQ score of approximately 70 or below. A score of 70 is two standard deviations below the average score for all people. Statistically, two standard deviations below average equal 2.5% of the population. However, DSM-5 (APA 2013) has moved away from relying on specific IQ scores. The estimates in DSM-5 report a prevalence rate of approximately one percent. IQ testing is discussed here.