Final answer:
Alfred Binet and Theodore Simon developed the Binet-Simon Intelligence Scale, which introduced the concept of mental age. Mental age is a measure of cognitive abilities in relation to chronological age.
Step-by-step explanation:
Alfred Binet and Theodore Simon were French psychologists who developed the Binet-Simon Intelligence Scale, the precursor to modern IQ tests. The concept of mental age was central to their work. Mental age is a measure of a person's cognitive abilities in relation to their chronological age. It is determined by comparing an individual's performance on a standardized intelligence test to the average performance of individuals in different age groups. For example, if a 10-year-old child performs at the level of an average 12-year-old on the test, their mental age would be 12.