mental age vs age

mental age and age both are nouns.

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

As nouns, age is a hypernym of mental age; that is, age is a word with a broader meaning than mental age:

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

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