Numbers 8:23

“And the Lord spake unto Moses, etc.] At the same time he gave the instructions about the consecration of the Levites, the time of their entrance on their service, and of leaving it, was fixed: saying; as follows.”

“8:23-26 Guidelines regarding the Levites’ length of service. • Whereas 4:3 states that their Tabernacle service began at age 30, 8:24 gives the age as 25. The discrepancy might reflect age requirements followed in different periods when the number of Levite men available for service varied, or those aged 25–29 may have been considered unofficial workers or apprentices. • Both passages establish the retirement age at 50; retired Levites could serve the Tabernacle as guards (cp. Ps 84:10).”

“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,…”