The Collaborative International Dictionary of English v.0.48:

Senile \Se"nile\, a. [L. senilis, from senex, gen. senis, old,
   an old man: cf. F. s['e]nile. See Senior.]
   Of or pertaining to old age; proceeding from, or
   characteristic of, old age; affected with the infirmities of
   old age; as, senile weakness. "Senile maturity of judgment."
   --Boyle.
   [1913 Webster]

   Senile gangrene (Med.), a form of gangrene occuring
      particularly in old people, and caused usually by
      insufficient blood supply due to degeneration of the walls
      of the smaller arteries.
      [1913 Webster]

WordNet (r) 3.0 (2006):

senile
    adj 1: mentally or physically infirm with age; "his mother was
           doddering and frail" [syn: doddering, doddery,
           gaga, senile]