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]

WordNet (r) 3.0 (2006):

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

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SENILITY. The state of being old.
     2. Sometimes in this state it is exceedingly difficult to know whether
the individual is or is not so deprived of the powers of his mind as to be
unable to manage his affairs. In general, senility of energy in some of the
intellectual operations, while the affections remain natural and
unperverted; such a state may, however, be followed by actual dementia or
idiocy.
     3. When on account of senility the party is unable to manage his
affairs, a committee will be appointed as in case of lunacy. 1 Coll. on
Lunacy, 66; 2 John. Ch. R. 232; 12 Ves. 446; 4 Call's R. 423; 5 John. Ch. R.
158; 8 Mass. 129; 2 Ves. sen. 407; 19 Ves. 285; 2 Cyclop. of Pract. Med.
872. See Aged Witness.