age vs senility

age and senility both are nouns.

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

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

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

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

age (noun)	senility (noun)
how long something has existed	the state of being senile
a time of life (usually defined in years) at which some particular qualification or power arises	mental infirmity as a consequence of old age; sometimes shown by foolish infatuations
a late time of life
an era of history having some distinctive feature
a prolonged period of time