Numbers 8:23

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

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

This is that which belongs 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;

and from the age of fifty years they shall cease waiting on the work, and shall serve no more,