Numbers 4:4

And the LORD spake unto Moses and unto Aaron, saying,

Take the sum of the sons of Kohath from among the sons of Levi, after their families, by the house of their fathers,

From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation.

And when the camp setteth forward, Aaron shall come, and his sons, and they shall take down the covering vail, and cover the ark of testimony with it: