Numbers 8:22-26

22After that, the Levites came to do their work at the tent of meeting under the supervision of Aaron and his sons. They did with the Levites just as the Lord commanded Moses. 23The Lord said to Moses, 24“This applies to the Levites: Men twenty-five years old or more shall come to take part in the work at the tent of meeting, 25but at the age of fifty, they must retire from their regular service and work no longer. 26They may assist their brothers in performing their duties at the tent of meeting, but they themselves must not do the work. This, then, is how you are to assign the responsibilities of the Levites.”