Numbers 1:3, 18, 20

from twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies.

and they assembled all the congregation together on the first day of the second month, and they declared their pedigrees after their families, by the house of their fathers, according to the number of the names, from twenty years old and upward, by their polls.

And the children of Reuben, Israel's eldest son, by their generations, after their families, by the house of their fathers, according to the number of the names, by their polls, every male from twenty years old and upward, all that were able to go forth to war