The Collaborative International Dictionary of English v.0.48:

Dotage \Do"tage\, n. [From Dote, v. i.]
   1. Feebleness or imbecility of understanding or mind,
      particularly in old age; the childishness of old age;
      senility; as, a venerable man, now in his dotage.
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            Capable of distinguishing between the infancy and
            the dotage of Greek literature.       --Macaulay.
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   2. Foolish utterance; drivel.
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            The sapless dotages of old Paris and Salamanca. --
                                                  Milton.
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   3. Excessive fondness; weak and foolish affection.
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            The dotage of the nation on presbytery. -- Bp.
                                                  Burnet.
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WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "dotage":
   advanced age, advanced years, age, age of retirement,
   an incurable disease, anility, blind faith, caducity, childishness,
   credulity, credulousness, debility, decline, decline of life,
   declining years, decrepitude, disposition to believe, dotardism,
   ease of belief, eld, elderliness, feebleness, fondness,
   green old age, gross credulity, hale old age, hoary age,
   infatuation, infirm old age, infirmity, infirmity of age,
   longevity, old age, oldness, overcredulity, overcredulousness,
   overopenness to conviction, overtrustfulness, pensionable age,
   rash conviction, ricketiness, ripe old age, second childhood,
   senectitude, senile debility, senile dementia, senile psychosis,
   senile weakness, senilism, senility, senior citizenship,
   superannuation, the downward slope, the golden years, trustfulness,
   uncritical acceptance, uncriticalness, unquestioning belief,
   unripe acceptation, unskepticalness, unsuspectingness,
   unsuspiciousness, vale of years, white hairs, will to believe,
   willingness to believe, wishful belief, wishful thinking