Numbers 1:3, 18, 20

From twenty years old and upward, all in Israel who are able to go to war, you and Aaron shall list them, company by company.

and on the first day of the second month, they assembled the whole congregation together, who registered themselves by clans, by fathers’ houses, according to the number of names from twenty years old and upward, head by head

The people of Reuben, Israel’s firstborn, their generations, by their clans, by their fathers’ houses, according to the number of names, head by head, every male from twenty years old and upward, all who were able to go to war