The Collaborative International Dictionary of English v.0.48:

Decrepitude \De*crep"i*tude\, n. [Cf. F. d['e]cr['e]pitude.]
   The broken state produced by decay and the infirmities of
   age; infirm old age.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decrepitude
    n 1: a state of deterioration due to old age or long use [syn:
         decrepitude, dilapidation]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "decrepitude":
   caducity, debilitation, debility, decay, deterioration,
   dilapidation, disease, dotage, enervation, feebleness, incapacity,
   infirmity, malaise, old age, ruin, senescence, senility,
   sickliness, superannuation, unhealthiness, weakness