Charles Spurgeon 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)

They were to take up this work as a warfare; for, though it was a peaceful work, yet it is described as a warfare: and whoever serves the Lord, though that service is perfect peace, will not serve Him without finding it to be a warfare as well.