bronze age vs period of time

bronze age and period of time both are nouns.

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

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

bronze age: (classical mythology) the third age of the world, marked by war and violence
period of time: an amount of time

Other hypernyms of bronze age include period, time period.

bronze age (noun)	period of time (noun)
(classical mythology) the third age of the world, marked by war and violence	an amount of time