Numbers 4:23
From thirty years old and upward until fifty years old shalt thou number them; all that enter in to perform the service, to do the work in the tabernacle of the congregation. From thirty years old and upward until fifty years old shalt thou number them; all that enter in to perform the service, to do the work in the tabernacle of the congregation.
The verse belongs to the instructions that assigned the Levites their service in the tabernacle. That service included erecting the tabernacle and encamping around it, and when Israel journeyed taking it down and carrying it together with the ark, the candlestick, and the other sacred furnishings. It sets thirty years old to fifty years old as the age range for those numbered to enter that service and do the work of the tabernacle of the congregation.
That work was given to designated Levites rather than left to anyone who chose it, as an orderly assignment treated as sacred. Those appointed were responsible for the sacred work of carrying and handling the tabernacle and its furnishings. The Levites were divided by family groups, each given a specific place and duty in caring for the sanctuary. Each worker was to perform only the work entrusted to him and not take up another's appointed task.