old age vs years

old age and years both are nouns.

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

As nouns, old age and years are synonyms defined as:

Other synonyms of old age include age, eld, geezerhood.

old age (noun)	years (noun)
a late time of life	a late time of life
	a prolonged period of time
	the time during which someone's life continues