Then the LORD spoke to Moses in the Wilderness of Sinai, saying,
The Levites being granted to Aaron to minister to him, they are here delivered to him by tale, that he might know what he had, and employ them accordingly. Observe, I. By what rule they were numbered: Every male from a month old and upward, Num 3:15.
Commenting on Numbers 3:14-39
And the Lord spake unto Moses in the wilderness of Sinai,.... At the same time he gave the order, and made the declaration before mentioned, and in the place where now the children of Israel were, and from whence they shortly removed: saying; as follows.
Number the children of Levi--They were numbered as well as the other tribes; but the enumeration was made on a different principle--for while in the other tribes the number of males was calculated from twenty years and upward [Num 1:3], in that of Levi they were counted "from a month old and upward." The reason for the distinction is obvious.