A person's mental age is the age which they are considered to have reached in their ability to think or reason.
mental age in British English
noun
psychology
the mental ability of a child, expressed in years and based on a comparison of his or her test performance with the performance of children with a range of chronological ages
See also intelligence quotient
mental age in American English
an individual's degree of mental development measured in terms of the chronological age of the average individual of corresponding mental ability