Numbers 8:24-26

New Revised Standard Version, Anglicised

²⁴ This applies to the Levites: from twenty-five years old and upwards 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 deal with the Levites in assigning their duties.