age vs mental age

age and mental age both are nouns.

Word	Noun	Adjective	Verb	Adverb
age	Yes	No	Yes	No
mental age	Yes	No	No	No

As nouns, mental age is a hyponym of age; that is, mental age is a word with a more specific, narrower meaning than age:

age: how long something has existed
mental age: the level of intellectual development as measured by an intelligence test

age (noun)	mental age (noun)
how long something has existed	the level of intellectual development as measured by an intelligence test
a time of life (usually defined in years) at which some particular qualification or power arises
a late time of life
an era of history having some distinctive feature
a prolonged period of time