In assigning the Levites (not of the priestly family of Aaron) to serve at the tent of meeting under the direction of the priests, Jehovah made loving provisions for their welfare. He commanded Moses: “This is what applies to the Levites: From twenty-five years old upward he will come to enter into the company in the service of the tent of meeting. But after the age of fifty years he will retire from the service company and serve no longer. And he must minister to his brothers in the tent of meeting in taking care of the obligation, but he must render no service.”​—Nu 8:23-26; 1Ch 23:3.

At Numbers chapter 4 the service organization of the Levites is described. There it is stated that they were to be registered from the ages of 30 to 50.

Evidently there was a five-year period from the ages of 25 to 30 years in which the Levite was serving in what may be termed “training.” It may have been that these younger ones were not used for the heavy duties, which were reserved for those 30 years and older​—full-grown men. (See AGE.) Later, after the Ark was permanently located on Mount Zion (and especially with the temple construction just ahead), the heavy work of carrying the sanctuary would no longer exist. David therefore arranged for the Levites to begin serving at the age of 20. Doubtless this was done because at the temple more would be needed to care for the greatly enlarged services there.​—1Ch 23:24-27.

The Levites who retired at the age of 50 did not retire from all service.