Numbers 4:26-30

and the curtains of the courtyard, the curtain for the entrance of the gate of the courtyard that is around the tabernacle and the altar, and their ropes and all the equipment for their service; and everything that is to be done by them, they shall perform.

All the service of the sons of the Gershonites, that is, all their loads and all their work, shall be performed at the command of Aaron and his sons; and you shall assign to them as a duty all their loads.

This is the service of the families of the sons of the Gershonites in the tent of meeting; and their duties shall be under the direction of Ithamar the son of Aaron the priest. Duties of the Merarites

“As for the sons of Merari, you shall count them by their families, by their fathers’ households;

from thirty years old and upward, even to fifty years old, you shall count them, everyone who can enter the service to do the work of the tent of meeting.