Numbers 8:23

21The Levites purified themselves from sin, and they washed their clothes; and Aaron offered them for a wave offering before the LORD and Aaron made atonement for them to cleanse them.

22After that, the Levites went in to do their service in the Tent of Meeting before Aaron and before his sons: as the LORD had commanded Moses concerning the Levites, so they did to them.

23The LORD spoke to Moses, saying,

24“This is what is assigned to the Levites: from twenty-five years old and upward they shall go in to wait on the service in the work of the Tent of Meeting;

25and from the age of fifty years they shall retire from doing the work, and shall serve no more,