NOUN (1)

1. someone whose age is in the sixties;


ADJECTIVE (1)

1. being from 60 to 69 years old;
- Example: "the sexagenarian population is growing"

The Collaborative International Dictionary of English v.0.48:

Sexagenarian \Sex`a*ge*na"ri*an\, n. [See Sexagenary.]
   A person who is sixty years old.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sexagenarian
    adj 1: being from 60 to 69 years old; "the sexagenarian
           population is growing"
    n 1: someone whose age is in the sixties