silver age vs period of time

silver age and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
silver age	Yes	No	No	No
period of time	Yes	No	No	No

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

silver age: (classical mythology) the second age of the world, characterized by opulence and irreligion; by extension, a period secondary in achievement to a golden age
period of time: an amount of time

Other hypernyms of silver age include period, time period.

silver age (noun)	period of time (noun)
(classical mythology) the second age of the world, characterized by opulence and irreligion; by extension, a period secondary in achievement to a golden age	an amount of time