John Gill Commentary Numbers 4:30

"from thirty years old and upward even unto fifty years old shalt thou number them, every one that entereth upon the service, to do the work of the tent of meeting." — Numbers 4:30 (ASV)

From thirty years old and upward
The Septuagint version here, and in (Numbers 4:3Numbers 4:23) , renders it, from twenty five years old and upward, agreeable to (Numbers 8:24) , at the which age the Levites went into the tabernacle, to wait and to learn their business, but they did not enter upon it till thirty;

even unto fifty years old shalt thou number them ;
at which age they were discharged from business:

everyone that entereth into the service, to do the work of the
tabernacle of the congregation; (See Gill on Numbers 4:3).