Numbers 4:21 BBE

¹⁹  But do this to them, so that life and not death may be theirs when they come near the most holy things; let Aaron and his sons go in and give to every one his work and that which he is to take up;

²⁰  But they themselves are not to go in to see the holy place, even for a minute, for fear of death.

²¹  And the Lord said to Moses,

²²  Let the sons of Gershon be numbered by families, in the order of their fathers' houses;

²³  All those from thirty to fifty years old who are able to do the work of the Tent of meeting.