Numbers 8:23-26

23The Lord said to Moses,

24“These are the instructions for the Levites: Men 25 years old or older are eligible to serve at the tent of meeting.

25But when they’re 50 years old, they must retire from active service and not work anymore.

26They may assist the other Levites in their duties at the tent of meeting, but they may not do any regular work. This is how you will handle the Levites’ duties.”