Numbers 4:23 in the World English Bible
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.
Context
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: