Numbers 3:22

Overview - Numbers 3
The sons of Aaron.
The Levites are given to the priests instead of the first-born;
14 are numbered by their families.
21 The families, number, and charge of the Gershonites;
27 of the Kohathites;
33 of the Merarites.
38 The place and charge of Moses and Aaron.
40 The first-born are freed by the Levites.
44 The overplus are redeemed.
Treasury of Scripture Knowledge

Numbers 3:22  (King James Version)
Those that were numbered of them, according to the number of all the males, from a month old and upward, even those that were numbered of them were seven thousand and five hundred.

from a month old
The males of all the other tribes were numbered from twenty years old and upwards; but, had the Levites been numbered in this way, they would not have been nearly equal in number to the first-born of the twelve tribes. Add to this, that as there must have been first-born of all ages in the other tribes, it was necessary that the Levites, who were to be their substitutes, should also be of all ages; and it appears to have been partly on this ground, that the Levites were numbered from a month old and upwards.