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]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "senile":
   aged, aging, ancient, anile, childish, childlike, crabbed,
   debilitated, declining, decrepit, doddered, doddering, doddery,
   doited, dotard, doting, dotty, enfeebled, failing, feeble, fogy,
   fogyish, forgetful, fossilized, fuddy-duddy, gerontal, gerontic,
   infirm, mossbacked, moth-eaten, mummylike, old, old fart, old fogy,
   old-fogy, old-fogyish, palsied, papery-skinned, ravaged with age,
   rickety, run to seed, rusty, senescent, shaky, shattered,
   shriveled, simple, stodgy, stricken in years, stuffy, timeworn,
   tottering, tottery, weak, withered, wizened