Numbers 1:3, 18, 20

from twenty years old and above—all who are able to go to war in Israel. You 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 recited their ancestry by families, by their fathers’ houses, according to the number of names, from twenty years old and above, each one individually.

Now the children of Reuben, Israel’s oldest son, their genealogies by their families, by their fathers’ house, according to the number of names, every male individually, from twenty years old and above, all who were able to go to war