From Longman Dictionary of Contemporary Englishmental ageˌmental ˈage noun [countable] MPTMa measure of someone’s ability to think, understand etc, expressed as the average age of a child with that level of ability a 25-year-old man with a mental age of seven
Examples from the Corpus
mental age• A person of 18 with Down's syndrome and a mental age of four, living with his or her parents?• A medical report estimated she had a mental age of seven years and nine months.• George Mayo is sixty two, but only has a mental age of six.• Children with a mental age of 3 years and above can understand the principle and benefit from its use.• Daughter Helen, 25, needed constant attention after whooping cough left her with a mental age of just two.• A.. There are some people whose physical and mental age is a whole lot younger than their nominal age.• Most of the boys' mental age is about eleven.• The most pathetic case was that of a middle-aged man with the mental age of a child of nine. 