C. F. Keil and Franz Delitzsch on Numbers 8:23-26

Num 8:23-26 The Levitical period of service is fixed here at twenty-five years of age and upwards to the fiftieth year. “This is what concerns the Levites,” i.e., what follows applies to the Levites. “From the age of twenty-five years shall he (the Levite) come to do service at the work of the tabernacle; and at fifty years of age shall he return from the service of the work, and not work any further, but only serve his brethren at the tabernacle in keeping charge,” i.e., help them to look after the furniture of the tabernacle. “Charge” (mishmereth), as distinguished from “work,” signified the oversight of all the furniture of the tabernacle (see Num 3:8); “work” (service) applied to laborious service, e.g., the taking down and setting up of the tabernacle and cleaning it, carrying wood and water for the sacrificial worship, slaying the animals for the daily and festal sacrifices of the congregation, etc.