1 Chronicles 23:3, 24-27

All the Levites who were thirty years old or older were counted, and the total came to 38,000.

These were the descendants of Levi by clans, the leaders of their family groups, registered carefully by name. Each had to be twenty years old or older to qualify for service in the house of the LORD. For David said, “The LORD, the God of Israel, has given us peace, and he will always live in Jerusalem. Now the Levites will no longer need to carry the Tabernacle and its furnishings from place to place.” In accordance with David’s final instructions, all the Levites twenty years old or older were registered for service.