How many letters does the top answer for “being from 90 to 99 years old” have?

The most common answer is nonagenarian (12 letters).

Crossword clue: being from 90 to 99 years old

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

being from 60 to 69 years old 100% similar → sexagenarian

being from 80 to 89 years old 100% similar → octogenarian

being at least 100 years old 75% similar → centenarian