1 Chronicles 23:3, 24, 27

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

These were the sons of Levi after their fathers’ houses, even the heads of the fathers’ houses of those of them that were counted, in the number of names by their polls, who did the work for the service of the house of Jehovah, from twenty years old and upward.

For by the last words of David the sons of Levi were numbered, from twenty years old and upward.