years vs autumn years

years and autumn years both are nouns.

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

As nouns, autumn years is a hyponym of years; that is, autumn years is a word with a more specific, narrower meaning than years:

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

Other hyponyms of years include dotage, second childhood, senility, decrepitude.

years (noun)	autumn years (noun)
a late time of life	(literary) the later years of one's life, especially after one has finished their career.
a prolonged period of time
the time during which someone's life continues