He started at an early age because it takes a long time to count that far.
Jokes aside, Avogadro never measured or calculated the value of Avogadro’s constant. The constant was named after him more than 50 years after his death, and the first measurements of the constant were around ten years after his death. Source: [Wikipedia](https://en.wikipedia.org/wiki/Avogadro_constant#First_measurements)
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