Charles Ellicott Commentary 1 Chronicles 23:27

"For by the last words of David the sons of Levi were numbered, from twenty years old and upward." — 1 Chronicles 23:27 (ASV)

For by the last words of David. —This means, because of his last commands. The Vulgate (juxta praecepta David novissima) and Syriac versions also understand it this way.

The Levites were numbered. —Literally, these (are), that is, according to the later idiom, this (is) the enumeration of the sons of Levi, from twenty years old and older.

The verse seems to mean that David, toward the end of his reign, ordered a census of Levites from twenty years old instead of thirty. The Authorized Version thus gives the sense. Others interpret it as, For in the Last words (that is, records) of David is the number of the sons of Levi from twenty, and so on, as if the chronicler were referring to some historical work in which this special census was recorded (Compare 1 Chronicles 29:29).

This verse is a parenthetical remark by the chronicler, interrupting David's speech, which, however, resumes in 1 Chronicles 23:28 and continues to the end of the chapter.