Numbers 8:23-26

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.”