iron age vs period of time

iron age and period of time both are nouns.

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

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

Other hypernyms of iron age include period, time period.

iron age (noun)	period of time (noun)
(classical mythology) the last and worst age of the world	an amount of time