Numbers 1:18-20

18 all the assembly also, called they together on the first of the second month, and they declared their pedigree according to their families by their ancestral houses, in the counting of names from twenty years old and upwards by their polls.

19 As Yahweh commanded Moses, so he mustered them in the desert of Sinai.

20 And they were as followeth The sons of Reuben, the firstborn of Israel, in their pedigrees by their families, by their ancestral houses, in the counting of names, by their polls, every male, from twenty years old and upwards, every one able to go forth to war;