Numbers 4:23 in the King James Version
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.
Context
22 “Take a census of the sons of Gershon also, by their fathers’ houses, by their families;
23 you shall count them from thirty years old and upward until fifty years old: all who enter in to wait on the service, to do the work in the Tent of Meeting.
24 “This is the service of the families of the Gershonites, in serving and in bearing burdens: