Mental retardation is not a specific disease, but rather a condition that can have a variety of causes; therefore, there is no specific time of onset for mental retardation. However, the classification is defined by an onset during childhood, specifically before the age of eighteen. If the onset is after the age of eighteen, the condition is formally classified as dementia even though the symptoms may be identical to what would qualify for mental retardation in a younger person. The distinction between dementia and mental retardation points to the reason the word 'retarded' is used; it suggests that affected individuals are unable to reach their potentials.
The cutoff point at age eighteen contained within the formal DSM-IV definition of mental retardation implies that cognitive abilities continue to develop until age eighteen. If the underlying cause of a person's mental retardation is due to an injury occurring before the age of eighteen, the injury retards the person from completing their full development; thus, the person has been retarded or delayed. If the mental retardation-causing injury occurs after the individual is eighteen years old, they are assumed to have already reached full development. Their condition is not considered one of retardation, but rather one of simple injury (e.g., brain damage) because full development of his mental abilities has already occurred.
The term dementia is used to refer to a deficit rather than a delay. There is no actual peak of intellectual development that occurs exactly on the individual's eighteenth birthday; the exact age is arbitrary, to an extent. From a neurological point of view, brain maturation and development does continue into the late teen years and early 20s. Eighteen years old is a reasonable cut-off point for distinguishing between dementia and mental retardation because at that age, the brain is nearly finished with its development in most cases.
It is possible for someone to have a dual diagnosis of mental and dementia at the same time. This might occur if a mentally retarded person sustains a brain injury after the age of eighteen.
