Mental age is a measure of intellectual development originally devised by Alfred Binet. It represents the chronological age that typically corresponds to a given level of test performance.

Binet and his colleague Théodore Simon created tasks of increasing difficulty and determined the average age at which children could pass each set. A child who could complete the items normally passed by ten-year-olds was said to have a mental age of ten, regardless of actual chronological age. The comparison of mental age with chronological age produced the original intelligence quotient: mental age divided by chronological age, multiplied by 100.

Modern intelligence tests have largely abandoned the mental-age concept in favor of deviation scores that compare an individual with age peers. Yet the underlying idea—that test performance can be referenced to developmental norms—remains. Mental age is still sometimes used in educational contexts to describe the approximate level at which a child is functioning.

Binet’s scale turned the observation of what children of different ages can do into a practical tool for identifying those who needed extra help.