IQ in the U.S. is 100. In the early days of IQ measurement, this was computed for children by dividing the mental age of an individual (as estim … ated by performance on a test) by the person's chronological age, and then multiplying that ratio by 100.o, a person with an average IQ cannot be diagnosed with mental retardation. Mental retardation is a generalized disorder, characterized by significantly impaired cognitive f … unctioning and deficits in two or more adaptive behaviors with onset before the age of 18.