1 Chronicles 23:3, 24-27

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

These were the sons of Levi according to their fathers’ households, even the heads of the fathers’ households of those of them who were counted, in the number of names by their census, doing the work for the service of the house of the LORD, from twenty years old and upward. For David said, “The LORD God of Israel has given rest to His people, and He dwells in Jerusalem forever. Also, the Levites will no longer need to carry the tabernacle and all its utensils for its service.” For by the last words of David the sons of Levi were numbered from twenty years old and upward.