http://www.uab.edu/cogdev/mentreta.htm defines mental retardation in terms of this definition of deficits in IQ as :
"Mental Deficiency (Retardation) proposed and adopted a three part definition in 1959. "Mental retardation refers to subaverage general intellectual functioning which originates in the developmental period and is associated with impairment in adaptive behavior" (Heber, 1961). Although this definition included the three components of low IQ (<85), impaired adaptive behavior, and origination before age 16, only IQ and age of onset were measurable with the existing psychometric techniques"
According to the Vineland social maturity scale deficits must also exist for one to be deemed mentaly retarded See http://www.come-over.to/FAS/VinelandCompare.htm
For such a diagnosis to be applied, "The diagnosis of mental retardation, for example, requires deficits in both cognitive ability and adaptive behavior, occuring before age 18. Adaptive behavior assessment is also used to determine the type and ...
Mental Retardation-intelligence and adaptive behaviors are discussed.