1 Chronicles 23:26 (KJV)

These were the sons of Levi after the house of their fathers; even the chief of the fathers, as they were counted by number of names by their polls, that did the work for the service of the house of the Lord, from the age of twenty years and upward.

For David said, The Lord God of Israel hath given rest unto his people, that they may dwell in Jerusalem for ever:

And also unto the Levites; they shall no more carry the tabernacle, nor any vessels of it for the service thereof.

For by the last words of David the Levites were numbered from twenty years old and above:

Because their office was to wait on the sons of Aaron for the service of the house of the Lord, in the courts, and in the chambers, and in the purifying of all holy things, and the work of the service of the house of God;