Numbers 3:14

“And the Lord spake unto Moses in the wilderness of Sinai, etc.] 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. In the other tribes the survey was made for purposes of war [Num 1:3], from which the Levites were totally exempt. But the Levites were appointed to a work on which they entered as soon as they were capable of instruction. They are mentioned under the names of Gershon, Kohath, and Merari, sons of Levi, and chiefs or ancestral heads of three subdivisions…”

“3:14-39 This first phase of a two-stage registration of the tribe of Levi (see 4:34-49 for stage two) summarized each family group’s special responsibilities (described in greater detail in 4:1-33). This numbering was not linked with military duties, so it enrolled male Levites from the age of one month (3:40-41). • Three family groups are descended from Gershon, Kohath, and Merari, the three sons of Levi. These clans (often called the Gershonites, Kohathites, and Merarites) later inherited the “cities of refuge” (Josh 21) and filled various important posts in the Jerusalem Temple (2 Chr 29:3-17).”

“The muster of the Levites included all the males from a month old and upwards, because they were to be sanctified to Jehovah in the place of the first-born; and it was at the age of a month that the latter were either to be given up or redeemed (comp. Num 3:40 and Num 3:43 with Num 18:16). In Num 3:17-20 the sons of Levi and their sons are enumerated, who were the founders of the mishpachoth among the Levites, as in Exo 6:16-19. Numbers 3:21”