Numbers 8:23-26

23The Lord also instructed Moses,

24“This is the rule the Levites must follow: They must begin serving in the Tabernacle at the age of twenty-five,

25and they must retire at the age of fifty.

26After 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.”