David Organizes the Levites
In addition to his preparations for the building of the temple, David also organizes the temple duties and personnel. This chapter introduces that organization. It deals primarily with those Levites who were to assist the priests in "the work of the service of the house of God" (verses 24, 28, 32), and gives a brief description of the work they were to do. It also points out specifically that the descendants of Moses were not given any special job beyond that of their fellow Kohathites (verses 14-17).
Note in verse 27 that David makes a change in how the Levites are to be numbered. Previously the Levites were not counted for service until age 30, apparently because the work of breaking down, transporting and setting up the tabernacle was considered too demanding and meticulous for younger men (verse 3; compare Numbers 4:2-3 Numbers 4:2-3  Take the sum of the sons of Kohath from among the sons of Levi, after their families, by the house of their fathers,
 From thirty years old and upward even until fifty years old, all that enter into the host, to do the work in the tabernacle of the congregation.
American King James Version×, Numbers 4:22-23 Numbers 4:22-23  Take also the sum of the sons of Gershon, throughout the houses of their fathers, by their families;  From thirty years old and upward until fifty years old shall you number them; all that enter in to perform the service, to do the work in the tabernacle of the congregation.
American King James Version×, Numbers 4:29-30 Numbers 4:29-30  As for the sons of Merari, you shall number them after their families, by the house of their fathers;  From thirty years old and upward even to fifty years old shall you number them, every one that enters into the service, to do the work of the tabernacle of the congregation.
American King James Version×). At the end of his life, David points out that this constant moving of the tabernacle will no longer be necessary (1 Chronicles 23:25-26 1 Chronicles 23:25-26  For David said, The LORD God of Israel has given rest to his people, that they may dwell in Jerusalem for ever:  And also to the Levites; they shall no more carry the tabernacle, nor any vessels of it for the service thereof.
American King James Version×), and he allows the Levites to enter into the temple service at age 20, the age of "adulthood" for the rest of the tribes.