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.

In accordance with David’s final instructions, all the Levites twenty years old or older were registered for service.