old age vs senility

old age and senility both are nouns.

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

As nouns, senility is a hyponym of old age; that is, senility is a word with a more specific, narrower meaning than old age:

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

Other hyponyms of old age include dotage, second childhood, autumn years, decrepitude.

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