Numbers 8:21-26

And the Levites purified themselves from sin and washed their clothes, and Aaron offered them as a wave offering before the LORD, and Aaron made atonement for them to cleanse them. And after that the Levites went in to do their service in the tent of meeting before Aaron and his sons; as the LORD had commanded Moses concerning the Levites, so they did to them. And the LORD spoke to Moses, saying, “This applies to the Levites: from twenty-five years old and upward they shall come to do duty in the service of the tent of meeting. And from the age of fifty years they shall withdraw from the duty of the service and serve no more. They minister to their brothers in the tent of meeting by keeping guard, but they shall do no service. Thus shall you do to the Levites in assigning their duties.”