How many letters does the top answer for “someone whose age is in the nineties” have?

The most common answer is nonagenarian (12 letters).

Crossword clue: someone whose age is in the nineties

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

nonagenarian 12 letters wn-def

someone whose age is in the nineties

someone whose age is in the eighties 50% similar → octogenarian

someone whose age is in the seventies 50% similar → septuagenarian

someone whose age is in the sixties 50% similar → sexagenarian