Alfred Binet (in collaboration with Theodore Simon) was instructed by the French government to design a test that would identify children who would have problems with school and or learning the material that was designed for children in their own age range. a widely used intelligence test.
They then set out to create a test that measures a child's mental age (the average mental ability for a child of a specific chronological age). They could use this mental age as a guide, to see if, for example, a 5 year old has "normal" intelligence, which would be a mental age of 5, or a mental age that was above or below. The test does not identify why children perform above or below a specific age range, only at what mental age a child performs. The test does however, according to its creators, measure how well a child of a specific age will be able to handle school work designed for children of similar age.