1 Chronicles 23:3, 24-27

Now the Levites were numbered from the age of thirty years and upward: and their number by their polls, man by man, was thirty and eight thousand.

These were the sons of Levi after the house of their fathers; even the chief of the fathers, as they were counted by number of names by their polls, that did the work for the service of the house of the LORD, from the age of twenty years and upward. For David said, The LORD God of Israel hath given rest unto his people, that they may dwell in Jerusalem for ever: and also unto the Levites; they shall no more carry the tabernacle, nor any vessels of it for the service thereof. For by the last words of David the Levites were numbered from twenty years old and above