decrepitude vs old age

decrepitude and old age both are nouns.

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

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

decrepitude: The quality or state of being wasted and weakened by or as if by the infirmities of old age.
old age: a late time of life

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

decrepitude (noun)	old age (noun)
a state of deterioration due to old age or long use	a late time of life
The quality or state of being wasted and weakened by or as if by the infirmities of old age.