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. (1 Chronicles 23:24)
According to Num. 4:3, 23, 30, 35, 43, 47, certain Levites began to serve at the age of 30, and according to Num. 8:24, 25, presumably another class of Levites began their service at the age of 25. Here, and again in 1 Chron. 23:27 and 2 Chron. 31:17, the age of 20 is mentioned. Men were enrolled for military service at the age of 20 (Num. 1:3). But in what way the Levites began their service at that age is not disclosed. Some think that some form of apprenticeship is involved. Others think that David lowered from 30 to 20 the age at which a Levite would begin his service.
These were the sons.
This is the subscription to the foregoing list of Levitical names.