Numbers 8:24-26

New Revised Standard Version Catholic Edition

²⁴ This applies to the Levites: from twenty-five years old and upward they shall begin to do duty in the service of the tent of meeting; ²⁵ and from the age of fifty years they shall retire from the duty of the service and serve no more. ²⁶ They may assist their brothers in the tent of meeting in carrying out their duties, but they shall perform no service. Thus you shall do with the Levites in assigning their duties.