How many letters does the top answer for “being at the best stage of development” have?

The most common answer is meridian (8 letters).

Crossword clue: being at the best stage of development

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

meridian 8 letters wn-def

an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the e

prime 5 letters wn-def

a natural number that has exactly two distinct natural number divisors: 1 and itself

being in the earliest stages of development 60% similar → rudimentary