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

The most common answer is sexagenarian (12 letters).

Crossword clue: someone whose age is in the sixties

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

sexagenarian 12 letters wn-def

someone whose age is in the sixties

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

someone whose age is in the nineties 50% similar → nonagenarian

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