The Collaborative International Dictionary of English v.0.48:

Senility \Se*nil"i*ty\, n. [Cf. F. s['e]nilit['e].]
   The quality or state of being senile; old age.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "senility":
   aboriginality, abulia, advanced age, advanced years, age,
   age of retirement, an incurable disease, ancien regime,
   ancientness, anility, antiquity, arteriosclerotic psychosis,
   atavism, caducity, certifiability, childishness,
   cobwebs of antiquity, debility, decline, decline of life,
   declining years, decrepitude, dementia paralytica, dipsomania,
   dotage, dotardism, drug addiction, dust of ages, eld, elderliness,
   eldership, feebleness, folie du doute, functional psychosis,
   general paralysis, general paresis, great age, green old age,
   hale old age, hoary age, hoary eld, incapacity, infirm old age,
   infirmity, infirmity of age, insipidity, instability, invalidism,
   inveteracy, longevity, metabolic psychosis, moral insanity,
   neurosis, old age, old order, old style, oldness,
   organic psychosis, paralytic dementia, pathological drunkenness,
   pensionable age, presenile dementia, primitiveness, primogeniture,
   primordialism, primordiality, prison psychosis, psychopathia,
   psychopathia sexualis, psychopathic condition,
   psychopathic personality, psychopathy, psychosis, ricketiness,
   ripe old age, second childhood, senectitude, senescence,
   senile debility, senile dementia, senile psychosis,
   senile weakness, senilism, senior citizenship, seniority,
   sexual pathology, shakiness, situational psychosis, superannuation,
   syphilitic paresis, the downward slope, the golden years,
   toxic psychosis, unfirmness, unsoundness, unsteadiness,
   unsturdiness, unsubstantiality, vale of years, vapidity,
   venerableness, wateriness, white hairs, wishy-washiness