autumn years vs old age

autumn years and old age both are nouns.

Word	Noun	Adjective	Verb	Adverb
autumn years	Yes	No	No	No
old age	Yes	No	No	No

As nouns, old age is a hypernym of autumn years; that is, old age is a word with a broader meaning than autumn years:

autumn years: (literary) the later years of one's life, especially after one has finished their career.
old age: a late time of life

Other hypernyms of autumn years include age, eld, geezerhood, years.

autumn years (noun)	old age (noun)
(literary) the later years of one's life, especially after one has finished their career.	a late time of life