Numbers 8:21-26

The Levites purified themselves from sin and washed their clothes, and Aaron lifted them up and presented them to the LORD as a special offering. He then offered a sacrifice to purify them and make them right with the LORD. After that the Levites went into the Tabernacle to perform their duties, assisting Aaron and his sons. So they carried out all the commands that the LORD gave Moses concerning the Levites. The LORD also instructed Moses, “This is the rule the Levites must follow: They must begin serving in the Tabernacle at the age of twenty-five, and they must retire at the age of fifty. After retirement they may assist their fellow Levites by serving as guards at the Tabernacle, but they may not officiate in the service. This is how you must assign duties to the Levites.”