“mental age n.: A child’s performance on a test of mental ability expressed as the average age of children who achieved the same level of performance in a standardization sample. Thus a 10-year-old child who achieves the same score as the average 12-year-old child in a standardization sample has a mental age of 12. The concept was introduced in 1905 by the French psychologists Alfred Binet (1857-1911) and Theodore Simon (1873-1961). See also Binet-Simon scale, IQ. Compare chronological age, social age. MA abbrev.”
Excerpted from: Colman, Andrew M., ed. Oxford Dictionary of Psychology. New York: Oxford University Press, 2003.