In Context

18 Cut ye not off the tribe of the families of the Kohathites from among the Levites:

19 But thus do to them, that they may live, and not die, when they approach to the most holy things: Aaron and his sons shall go in, and appoint them every one to his service and to his burden:

20 But they shall not go in to see when the holy things are covered, lest they die.

21 And the LORD spoke to Moses, saying,

22 Take also the sum of the sons of Gershon, throughout the house of their fathers, by their families;

23 From thirty years old and upward until fifty years old shalt thou number them; all that enter in to perform the service, to do the work in the tabernacle of the congregation.

Numbers 4:21 in Other Translations