Source : Webster's Revised Unabridged Dictionary (1913)

Dotard \Do"tard\, n. [{Dote}, v. i.]
   One whose mind is impaired by age; one in second childhood.

         The sickly dotard wants a wife.          -- Prior.

Source : WordNet®

dotard
     n : an oldster in his dotage; someone whose age has impaired his
         intellect