Numbers 8:22-26

MLV(i) 22 And after that the Levites went in to do their service in the tent of meeting before Aaron and before his sons. As Jehovah had commanded Moses concerning the Levites, so they did to them.
23 And Jehovah spoke to Moses, saying, 24 This is what belongs to the Levites: From twenty-five years old and upward they will go in to wait upon the service in the work of the tent of meeting. 25 And from the age of fifty years they will cease waiting upon the work and will serve no more, 26 but will minister with their brothers in the tent of meeting, to keep the charge and will do no service. Thus will you do to the Levites concerning their charges.