Numbers 8:20-24

So this is what Moses, Aaron, and all the congregation of the sons of Israel did to the Levites; according to everything that the Lord had commanded Moses regarding the Levites, so the sons of Israel did to them.

The Levites, too, purified themselves from sin and washed their clothes; and Aaron presented them as a wave offering before the Lord. Aaron also made atonement for them to cleanse them.

Then after that the Levites went in to perform their service in the tent of meeting before Aaron and his sons; just as the Lord had commanded Moses concerning the Levites, so they did to them. Retirement for the Levites

Now the Lord spoke to Moses, saying,

“This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform service in the work of the tent of meeting.