We very commonly use the term "Mentally retarded" for any sort of developmental or behavior disorder. That's very surface level judgement. Actually we categorize all sorts of brain disorder victims like down syndrome, cerebral palsy, pervasive developmental disorder, autistic disorder, etc. in same banner. But fact is not as we think of it. A person can be diagnosed as mentally retarded only when all the criteria listed in the attached pic will be present I'm his/her behavior. In that sense mental retardation is much deeper problem than what we can realize.
In 2003, epidemiological studies showed that about 1 to 3 in every 100 individuals in USA was mentally retarded (Wicks-Nelson & Israel). The central characteristic of mental retardation is subaverage intellectual functioning that begin before age of 18 & exist together with significant limitations in adaptive functioning. These limitations may occur in two or more of a wide variety of basic skill areas. It is important to note that these criteria include more than intelligence test scores, they also emphasize adaptive behavior. Thus, people who score in the retarded range on intelligence tests but otherwise get along adequately at home, school or work are not classified as mentally retarded.
DSM-IV-TR divided mental retardation in 4 levels - mild, moderate, severe & profound, on the basis of intelligence test score. IQ level 50-55 to 70 is mild, 35-40 to 50-55 moderate, 20-25 to 35-40 severe & below 20 or 25 is profound. If a diagnosis of mental retardation is made, it's entered into AXIS II of DSM-IV-TR. If the person also meets the criteria of some psychopathology like autism, bipolar disorder or schizophrenia, that diagnosis is made on Axis I. Though it was believed that people with mental retardation don't have Axis I disorders, but now we know that 2 types of condition can exist together.