Numbers 4:3
From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation. From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation.
From thirty years old and upward even until fifty years old was the age range for entering the host to do the work in the tabernacle of the congregation. That entering into the host for the work was assigned to the Levites. The appointment is applied to the Kohathites, whom God appointed to duties involving the tabernacle and its most holy things. It belongs to instructions that assigned sacred tabernacle work to designated people rather than leaving it to any tribe or volunteer, showing the service was organized and specifically appointed.
That Levite service included erecting the tabernacle and encamping around it and, when Israel journeyed, taking it down and carrying it with the ark, candlestick, and other sacred furnishings. This service was an honor given to the Levites for their faithfulness concerning the golden calf and God's command against the idolaters. The arrangement belongs to God's divinely appointed order for the work, given as a lesson for His people to follow.