blue moon vs age

blue moon and age both are nouns.

Word	Noun	Adjective	Verb	Adverb
blue moon	Yes	No	No	No
age	Yes	No	Yes	No

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

Other hypernyms of blue moon include long time, years.

blue moon (noun)	age (noun)
a long time	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