Numbers 4:3

“So when the tabernacle is to set out, the Levites shall take it down; and when the tabernacle encamps, the Levites shall set it up. But the layman who comes near shall be put to death.

from thirty years and upward to fifty years old, you shall number them; all who enter to perform the duty of performing the service in the tent of meeting.

from thirty years and upward even to fifty years old, you shall number them, everyone who enters the duty of performing the service of the tent of meeting.

“This is what applies to the Levites: from twenty-five years old and upward they shall enter to perform their duty in the service of the tent of meeting.

Now David was thirty years old when he became king, and he reigned forty years.

And the Levites were numbered from thirty years old and upward, and their number by census of men was 38,000.

“Yet they shall be ministers in My sanctuary, having oversight at the gates of the house and ministering in the house; they shall slaughter the burnt offering and the sacrifice for the people, and they shall stand before them to minister to them.