Albert Barnes Commentary 1 Chronicles 23:24

"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." — 1 Chronicles 23:24 (ASV)

From the age of twenty years - Previously, the Levites did not enter upon their regular functions until the age of thirty (1 Chronicles 23:3). While the Law imposed certain lighter duties on them at the age of twenty-five (Numbers 8:24), it was not until they were five years older that they became responsible for the full service of the sanctuary.

David, however, appears to have made a change. According to his “last words” (1 Chronicles 23:27), the time for the Levites to enter the full duties of their office was advanced from thirty to twenty years.

This change was based on the lighter character of their labors, now that the ark had ceased to be carried from place to place and had obtained a permanent habitation (1 Chronicles 23:26). The age limit David fixed continued in later times .