golden age vs heyday

golden age and heyday both are nouns.

Word	Noun	Adjective	Verb	Adverb
golden age	Yes	No	No	No
heyday	Yes	No	No	No

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

Other hypernyms of golden age include bloom, blossom, efflorescence, flower, flush, peak, prime.

golden age (noun)	heyday (noun)
any period (sometimes imaginary) of great peace and prosperity and happiness	the period of greatest prosperity or productivity
a time period when some activity or skill was at its peak