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. Twenty years and upward - It appears that this was a different ordinance from that mentioned 1 Chronicles 23:3. At first he appointed the Levites to serve from thirty years and upward; now from twenty years. These were David's last orders; see 1 Chronicles 23:27. They should begin at an earlier age, and continue later.
This was not a very painful task; the ark being now fixed, and the Levites very numerous, there could be no drudgery.