The Collaborative International Dictionary of English v.0.48:

Septuagenarian \Sep`tu*a*ge*na"ri*an\, n.
   A person who is seventy years of age; a septuagenary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

septuagenarian
    n 1: someone whose age is in the seventies

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "septuagenarian":
   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, 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, presbyter, quindecennial, quindecim,
   quindecima, quindene, score, senior citizen, 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