Numbers 8:23

19 And I have given the Levites as a gift to Aaron and his sons from among the Israelites, to perform the service for the Israelites at the Tent of Meeting and to make atonement on their behalf, so that no plague will come against the Israelites when they approach the sanctuary.”

20 So Moses, Aaron, and the whole congregation of Israel did with the Levites everything that the LORD had commanded Moses they should do.

21 The Levites purified themselves and washed their clothes, and Aaron presented them as a wave offering before the LORD. Aaron also made atonement for them to cleanse them.

22 After that, the Levites came to perform their service at the Tent of Meeting in the presence of Aaron and his sons. Thus they did with the Levites just as the LORD had commanded Moses.

23 And the LORD said to Moses,

24 “This applies to the Levites: Men twenty-five years of age or older shall enter to perform the service in the work at the Tent of Meeting.

25 But at the age of fifty, they must retire from performing the work and no longer serve.

26 After that, they may assist their brothers in fulfilling their duties at the Tent of Meeting, but they themselves are not to do the work. This is how you are to assign responsibilities to the Levites.”

Read all of Numbers 8