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]