old age vs eld

old age and eld both are nouns.

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

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

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

old age (noun)	eld (noun)
a late time of life	a late time of life
	a time of life (usually defined in years) at which some particular qualification or power arises