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 combining form of sex (see six) + -genaria "ten times," from -ginta "tens," from PIE *dkm-ta-, from root *dekm- "ten." As an adjective from 1836.