The Collaborative International Dictionary of English v.0.48:

Octogenary \Oc*tog"e*na*ry\, a. [L. octogenarius, from octogeni
   eighty each, octoginta eighty, fr. octo eight. See Eight,
   Eighty.]
   Of eighty years of age. "Being then octogenary." --Aubrey.
   [1913 Webster]