NOUN (1)

1. someone whose age is in the nineties;


ADJECTIVE (1)

1. being from 90 to 99 years old;
- Example: "the nonagenarian inhabitants of the nursing home"

The Collaborative International Dictionary of English v.0.48:

Nonagenarian \Non`a*ge*na"ri*an\, n. [L. nonagenarius
   containing, or consisting of, ninety, fr. nonageni ninety
   each; akin to novem nine.]
   A person ninety years old.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonagenarian
    adj 1: being from 90 to 99 years old; "the nonagenarian
           inhabitants of the nursing home"
    n 1: someone whose age is in the nineties

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "nonagenarian":
   Father Time, L, Methuselah, Nestor, Old Paar, Sexagesima, boxcar,
   boxcars, centenarian, dotard, dozen, duodecimo, eighty, elder,
   eleven, fifteen, fifty, five and twenty, fortnight, forty,
   four and twenty, fourscore, fourscore and ten, fourteen, gaffer,
   geezer, golden-ager, gramps, grandfather, grandsire, graybeard,
   half a hundred, long dozen, ninety, 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, presbyter, quindecennial, quindecim,
   quindecima, quindene, score, senior citizen, septuagenarian,
   seventy, sexagenarian, sexagenary, sexagesimo-quarto, sixteen,
   sixteenmo, sixty, sixty-four, sixty-fourmo, teens,
   the quiet-voiced elders, thirteen, thirty-two, thirty-twomo,
   threescore, threescore and ten, twelve, twelvemo, twenty,
   twenty-five, twenty-four, twenty-fourmo, two dozen, two weeks,
   twoscore, venerable sir, veteran