Mental age is a concept related to intelligence. It looks at how a specific individual, at a specific age, performs intellectually, compared to average intellectual performance for that individual's actual chronological age (i.e. time elapsed since birth). The intellectual performance is based on performance in tests and live assessments by a psychologist. The score achieved by the individual is compared to the median average scores at various ages, and the mental age (x, say) is derived such that the individual's score equates to the average score at age x.

However, mental age depends on what kind of intelligence is measured. For instance, a child's intellectual age can be average for their actual age, but the same child's emotional intelligence can be immature for their physical age. Psychologists often remark that girls are more emotionally mature than boys at around the age of puberty. Also, a six-year-old child intellectually gifted can remain a three-year-old child in terms of emotional maturity. Mental age can be considered a controversial concept.