Also found in: Dictionary, Thesaurus, Encyclopedia. 5 million mentally retarded the development of children with mental retardation live in the United States alone. Mental retardation begins in childhood or adolescence before the age of 18.
In most cases, it persists throughout adulthood. A diagnosis of mental retardation is made if an individual has an intellectual functioning level well below average and significant limitations in two or more adaptive skill areas. In general, mentally retarded children reach developmental milestones such as walking and talking much later than the general population. Symptoms of mental retardation may appear at birth or later in childhood.