nonagenarian (n.)

"person who has reached 90 years old; person between 90 and 100 years old;" 1776, coined in English with -an + Latin nonagenarius "containing ninety" (in Late Latin "someone 90 years old"), from nonagen "ninety each," related to nonaginta "the number ninety," from nonus "ninth" (from novem "nine;" see nine) + -genaria "ten times," from PIE *dkm-ta-, from root *dekm- "ten." As an adjective, "of age 90 to 100," by 1867.