I Chronicles 23:3, 24, 26-27

Now the Levites were numbered from the age of thirty years and above; and the number of individual males was thirty-eight thousand.

These were the sons of Levi by their fathers’ houses—the heads of the fathers’ houses as they were counted individually by the number of their names, who did the work for the service of the house of the LORD, from the age of twenty years and above.

and also to the Levites, “They shall no longer carry the tabernacle, or any of the articles for its service.” For by the last words of David the Levites were numbered from twenty years old and above