Numbers 3:15

Verse Numbers 3:15. A month old and upward] The males of all the other tribes were numbered, from twenty years and upward; had the Levites been numbered in this way, they would not have been nearly equal in number to the firstborn 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 be also of all ages; and it appears to have been on this ground, at least partly, that the Levites were numbered from four weeks old and upward.