The history of "mental age"
The concept of mental age was introduced by Alfred Binet in 1905 as part of the Binet-Simon scale — the precursor to modern IQ tests. Binet defined mental age as the level of intellectual functioning corresponding to a particular chronological age, based on performance on a specific battery of tasks.
By the mid-20th century, the concept fell out of mainstream use. The limitations were clear: intelligence is multidimensional (not a single number), cultural and educational background heavily influence test scores, and adult intelligence doesn't meaningfully "age" the way the concept implied. Modern psychology uses more nuanced measures like composite cognitive scores, domain-specific assessments, and fluid/crystallised intelligence frameworks.
The popular "mental age quiz" genre on the internet bears no relationship to the original Binet concept or modern cognitive science. These quizzes match personality preferences to stereotyped age profiles for entertainment, not measurement.