The Collaborative International Dictionary of English v.0.48:

Oldster \Old"ster\, n. [Cf. Youngster.]
   An old person. [Jocular] --H. Kingsley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oldster
    n 1: an elderly person [syn: oldster, old person, senior
         citizen, golden ager]