Numbers 8:23-26

23The Lord spoke to Moses:

24“In regard to the Levites: From twenty-five years old or more, a man enters the service in the work at the tent of meeting.

25But at fifty years old he is to retire from his service in the work and no longer serve.

26He may assist his brothers to fulfill responsibilities at the tent of meeting, but he must not do the work. This is how you are to deal with the Levites regarding their duties.”