Then the Lord spoke to Moses, saying, “Take a census of the sons of Gershon also, by their fathers’ households, by their families. From thirty years and upward to fifty years old, you shall number them; all who enter for service to do the work in the Tent of Meeting. This is the service of the families of the Gershonites, in serving and in carrying [when on the march]: they shall carry the tent curtains of the tabernacle, and the Tent of Meeting, its covering and the covering of porpoise skin (fine leather) that is on top of it, and the curtain for the doorway of the Tent of Meeting (tabernacle), and the hangings of the courtyard, and the curtain for the entrance of the gate of the courtyard which is around the tabernacle and the altar, and their tent ropes, and all the equipment for their service; and all that is to be done, they shall perform. Aaron and his sons shall direct all the service of the sons of the Gershonites, all they have to carry and all they have to do. You shall assign to them as a duty all that they are to carry [on the march]. This is the service of the families of the sons of Gershon in the Tent of Meeting (tabernacle); and their duties shall be under the direction of Ithamar the son of Aaron, the [high] priest.
Numbers 4:21-28 (AMP)
The next family from the Levitical tribe, the Gershonites, handled the various coverings, cords, and hangings that adorned the tabernacle. In keeping with the latest census, only those thirty to fifty years of age could actually serve in this capacity.
The restriction came not because of some ceremonial reason or need for men in the full vigor of strength (for those in their twenties are, for the most part, stronger than those in their thirties), but because the margin of error was so small in these sacred undertakings.
The prescribed age bracket provided a hedge against errors stemming from physical limitation (i.e., those about fifty) and those attributed to insufficient experience and maturation (those under thirty).
Holman Old Testament Commentary – Numbers