Numbers 4:30

“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, even to fifty years old, all who enter the duty of doing the work in the tent of meeting.

Then the tabernacle was taken down; and the sons of Gershon and the sons of Merari, who were carrying the tabernacle, set out.

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.