old boy vs golden ager

old boy and golden ager both are nouns.

Word	Noun	Adjective	Verb	Adverb
old boy	Yes	No	No	No
golden ager	Yes	No	No	No

As nouns, golden ager is a hypernym of old boy; that is, golden ager is a word with a broader meaning than old boy:

Other hypernyms of old boy include old person, oldster, senior citizen.

old boy (noun)	golden ager (noun)
a vivacious elderly man	an elderly person
a former male pupil of a school
a familiar term of address for a man