24 These were the descendants of Levi by their families—the heads of families as they were registered under their names and counted individually, that is, the workers twenty years old or more who served in the temple of the LORD.

Read 1 Chronicles 23:24 Using Other Translations

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.
These were the sons of Levi by their fathers' houses, the heads of fathers' houses as they were listed according to the number of the names of the individuals from twenty years old and upward who were to do the work for the service of the house of the LORD.
These were the descendants of Levi by clans, the leaders of their family groups, registered carefully by name. Each had to be twenty years old or older to qualify for service in the house of the LORD .