second childhood vs eld

second childhood and eld both are nouns.

Word	Noun	Adjective	Verb	Adverb
second childhood	Yes	No	No	No
eld	Yes	No	No	No

As nouns, eld is a hypernym of second childhood; that is, eld is a word with a broader meaning than second childhood:

second childhood: mental infirmity as a consequence of old age; sometimes shown by foolish infatuations
eld: a late time of life

Other hypernyms of second childhood include age, geezerhood, old age, years.

second childhood (noun)	eld (noun)
mental infirmity as a consequence of old age; sometimes shown by foolish infatuations	a late time of life
	a time of life (usually defined in years) at which some particular qualification or power arises