2, 3. sons of Kohath, from thirty years old and upward--This age was specifically fixed (see on Nu 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 (Nu 8:26).
      all that enter into the host--so called from their number, the order and discipline maintained through their ranks, and their special duty as guards of the tabernacle. The Hebrew word, however, signifies also a station or office; and hence the passage may be rendered, "All that enter into the sacerdotal office" (Nu 4:23).

JFB.