The Collaborative International Dictionary of English v.0.48:

Elderly \Eld"er*ly\, a.
   Somewhat old; advanced beyond middle age; bordering on old
   age; as, elderly people.
   [1913 Webster]

WordNet (r) 3.0 (2006):

elderly
    adj 1: advanced in years; (`aged' is pronounced as two
           syllables); "aged members of the society"; "elderly
           residents could remember the construction of the first
           skyscraper"; "senior citizen" [syn: aged, elderly,
           older, senior]
    n 1: people who are old collectively; "special arrangements were
         available for the aged" [syn: aged, elderly] [ant:
         young, youth]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "elderly":
   advanced, advanced in life, advanced in years, age-old, aged,
   ageing, ageless, aging, along in years, ancient, anile, antique,
   auld, dateless, declining, decrepit, gray, gray with age,
   gray-haired, gray-headed, grey, grown old, hoar, hoary, immemorial,
   of old, of yore, old, old as Methuselah, old as history,
   old as time, old-fogyish, old-time, olden, over the hill,
   patriarchal, pensioners, senectuous, senescent, senile,
   superannuated, timeless, venerable, white, white with age,
   white-bearded, white-crowned, white-haired, wrinkled, wrinkly,
   years old