Numbers 4:1

“And the Lord spake unto Moses, and unto Aaron, etc.] After they had taken the number of the Levites, and made an exchange of the firstborn of Israel for them; and now they are ordered to number them a second time, and take out from them such as were fit for service: saying; as follows.”

“OF THE LEVITES' SERVICE. (Num. 4:1-49) sons of Kohath, from thirty years old and upward--This age was specifically fixed (see on Num 8:24) as the full maturity of bodily energy to perform the laborious duties assigned them in the wilderness, as well as of mental activity to assist in the management of the sacred services. And it was the period of life at which John the Baptist and Christ entered on their respective ministries. even unto fifty--The term prescribed for active duty was a period of twenty years, at the end of which they were exempted from the physical labors of the office, though still expected to attend in the tabernacle (Num 8:26). all that enter into the host--so called from…”

“4:1-49 This section expands the discussion begun in 3:21-37 on the duties of the three clans of Levites. The Levites—on the march and encamped—carried and cared for the Tabernacle and were always surrounded by the other tribes so that they and their precious cargo would be safe (2:17). The sacred things associated with God were placed in the middle of the tribes, and the role of the priests and their Levitical assistants was to keep them holy by keeping them separate. • God, in his holiness, is worthy of attention to the details of his will. His Tabernacle and its furnishings were sacred, and the tribe of Levi was responsible to maintain the distinction between holy and common, between clean…”

“Rules of Service, and Numbering of the Levites Qualified for Service. - After the adoption of the Levites for service at the sanctuary, in the place of the first-born of Israel, Moses and Aaron mustered the three families of the Levites by the command of God for the service to be performed by those who were between the ages of 30 and 50. The particulars of the service are first of all described in detail (vv. 4-33); and then the men in each family are taken, of the specified age for service (vv. 34-49). The three families are not arranged according to the relative ages of their founders, but according to the importance or sacredness of their service. The Kohathites take the lead, because the…”