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Mental age is a concept in relation to intelligence, expressed as the age at which a child is performing intellectually. The mental age of the child that is tested is the same as the average age at which normal children achieve a particular score.
However, a mental age result on an intelligence test does not mean that children function at their "mental age level" in all aspects of life. For instance, a gifted six-year-old child can still in some ways function as a three-year-old child. Mental age was once considered a controversial concept.
Originally, the differences between mental age and chronological age were used to compute the intelligence quotient, or IQ. This was computed using the ratio method, with the following formula: mental age/chronological age * 100 = IQ. No matter what the child's chronological age, if the mental age is the same as the chronological age, then the IQ will equal 100.
An IQ of 100 thus indicates a child of average intellectual development. For a gifted child, the mental age is above the chronological age, and the IQ is higher than 140; for an intellectually challenged child, the mental age is below the chronological age, and the IQ is below 70.