sexagenarian (n.)
1738, "person sixty years old," from Latin sexagenarius "containing sixty," from sexagenarius, from sexageni "sixty each, sixty at a time," from sexaginta "sixty," from comb. form of sex (see six) + -genaria "ten times," from -ginta "tens," from PIE *dkm-ta-, from *dekm- "ten" (see ten). As an adjective from 1836.