Numbers 4:3-5

Amplified Bible, Classic Edition

³ From thirty years old and up to fifty years old, all who can enter the service to do the work in the Tent of Meeting.

⁴ This shall be the responsibility of the sons of Kohath in the Tent of Meeting: the most holy things.

⁵ When the camp prepares to set forward, Aaron and his sons shall take down the veil [screening the Holy of Holies] and cover the ark of the Testimony with it,