Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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