John Gill 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
Or therefore by the last orders he gave before his death:

the Levites were numbered from twenty years old and above ;
as able and sufficient to do the work of their office.