age vs Dark Ages

age and Dark Ages both are nouns.

Word	Noun	Adjective	Verb	Adverb
age	Yes	No	Yes	No
Dark Ages	Yes	No	No	No

age (noun)	Dark Ages (noun)
how long something has existed	the period of history between classical antiquity and the Italian Renaissance
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