WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "senior citizen":
   Father Time, Methuselah, Nestor, Old Paar, ancient, centenarian,
   dotard, elder, gaffer, geezer, golden-ager, gramps, grandfather,
   grandsire, graybeard, nonagenarian, octogenarian, old chap,
   old codger, old dog, old duffer, old geezer, old gent,
   old gentleman, old man, old party, old-timer, older, oldster,
   pantaloon, patriarch, pensioner, presbyter, senior, septuagenarian,
   sexagenarian, the quiet-voiced elders, venerable sir, veteran