How many letters does the top answer for “a very old person” have?

The most common answer is antediluvian (12 letters).

Crossword clue: a very old person

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

antediluvian 12 letters wn-def

a very old person

a man who is very old 50% similar → graybeard, methuselah

a person who is very poor 50% similar → pauper

a very brave person. 50% similar → lionheart