Source : Webster's Revised Unabridged Dictionary (1913)

Senility \Se*nil"i*ty\, n. [Cf. F. s['e]nilit['e].]
   The quality or state of being senile; old age.

Source : WordNet®

senility
     n 1: mental infirmity as a consequence of old age; sometimes
          shown by foolish infatuations [syn: {dotage}, {second
          childhood}]
     2: the state of being senile