1 Chronicles 23:3, 24-27

The Levites thirty years old or more were counted, and the total number of men was thirty-eight thousand.

These were the descendants of Levi by their families—the heads of families as they were registered under their names and counted individually, that is, the workers twenty years old or more who served in the temple of the LORD. For David had said, “Since the LORD, the God of Israel, has granted rest to his people and has come to dwell in Jerusalem forever, the Levites no longer need to carry the tabernacle or any of the articles used in its service.” According to the last instructions of David, the Levites were counted from those twenty years old or more.