Numbers 4:21

¹⁹  But thus do unto them, that they may live, and not die, when they approach unto 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 spake unto Moses, saying,

²²  Take also the sum of the sons of Gershon, throughout the houses 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.