Adam Clarke on 1 Chronicles 23:24

Twenty years and upward - It appears that this was a different ordinance from that mentioned 1Chr 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 1Chr 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.