senility vs old age

senility and old age both are nouns.

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

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

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

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

senility (noun)	old age (noun)
the state of being senile	a late time of life
mental infirmity as a consequence of old age; sometimes shown by foolish infatuations