Numbers 4:22-24

King James Version

²² Take also the sum of the sons of Gershon, throughout the houses of their fathers, by their families;

²³ From thirty years old and upward until fifty years old shalt thou number them; all that enter in to perform the service, to do the work in the tabernacle of the congregation.

²⁴ This is the service of the families of the Gershonites, to serve, and for burdens: