Mental age

Mental age is a controversial concept in psychometrics. It is an intelligence test score, expressed as the chronological age for which a given level of performance is average or typical.

How mental age is derived
After taking a standardized test, an individual's mental age is divided by his chronological age and multiplied by 100, yielding an intelligence quotient (IQ). Thus, a subject whose mental and chronological ages are identical has an IQ of 100, or average intelligence.

Louis Leon Thurstone was among prominent critics, stating "the mental age concept is a failure in that it leads to ambiguities and inconsistencies."