The Collaborative International Dictionary of English v.0.48:

Gerontocracy \Ger`on*toc"ra*cy\, n. [Gr. ?, ?, an old man + ? to
   rule.]
   Government by old men. [R.] --Gladstone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gerontocracy
    n 1: a political system governed by old men