1 Kings 2:11

The time that David reigned over Israel was forty years: he reigned seven years in Hebron and thirty-three years in Jerusalem.

David was thirty years old when he became king, and he reigned forty years.

Thus David the son of Jesse reigned over all Israel.

Treasury of Scripture Knowledge

And the days that David reigned over Israel were forty years: seven years reigned he in Hebron, and thirty and three years reigned he in Jerusalem.

David was thirty years old when he became king, and he reigned forty years.

Thus David the son of Jesse reigned over all Israel.