The Collaborative International Dictionary of English v.0.48:

Caducity \Ca*du"ci*ty\, n. [LL. caducitas: cf. F. caducit['e].
   See Caducous.]
   Tendency to fall; the feebleness of old age; senility. [R.]
   [1913 Webster]

         [A] jumble of youth and caducity.        --Chesterfield.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "caducity":
   advanced age, advanced years, age of retirement,
   an incurable disease, anility, changeableness, childishness,
   corruptibility, death, debility, decline, decline of life,
   declining years, decrepitude, dotage, dotardism, eld, elderliness,
   ephemerality, ephemeralness, evanescence, feebleness, finitude,
   fleetingness, fugacity, green old age, hale old age, hoary age,
   impermanence, impermanency, incapacity, infirm old age, infirmity,
   infirmity of age, insipidity, instability, invalidism, longevity,
   momentariness, mortality, mutability, old age, oldness,
   pensionable age, perishability, ricketiness, ripe old age,
   second childhood, senectitude, senile debility, senile dementia,
   senile psychosis, senile weakness, senilism, senility,
   senior citizenship, shakiness, superannuation, the downward slope,
   the golden years, transience, transiency, transientness,
   transitoriness, unfirmness, unsoundness, unsteadiness,
   unsturdiness, unsubstantiality, vale of years, vapidity,
   volatility, wateriness, white hairs, wishy-washiness