Charles Ellicott Commentary Numbers 4:3

"from thirty years old and upward even until fifty years old, all that enter upon the service, to do the work in the tent of meeting." — Numbers 4:3 (ASV)

From thirty years old and upward even until fifty years old.—The previous census of the Levites was from a month old. The present census was for the purpose of discharging duties requiring considerable physical strength; therefore, the prescribed age for entering upon these duties was fixed at that time at thirty and limited to fifty.

Some have supposed that five years were spent in preparation for the service, and that this is how the apparent discrepancy between this verse and Numbers 8:24 (where the age for entering the service is fixed at twenty-five) is to be reconciled. (See Note on Numbers 8:24.) In Eastern countries, physical strength declines at an earlier age than in colder, more temperate climates. Thirty was the age at which John the Baptist and our Lord began their public ministry.

All that enter into the host.—Or, everyone who enters upon the service. The word zaba, commonly translated as host, and used elsewhere to signify military service, is here used to signify the service of the sanctuary.