Asa his son reigned - If Rehoboam was (1 Kings 12:8 note) not more than 21 years old at his accession, Asa, when he mounted the throne, must have been a mere boy, not more than 10 or 11 years of age.
The land was quiet ten years - The great blow struck by Abijah 2 Chronicles 13:15-19, his alliance with Syria 1 Kings 15:19, and the rapid succession of sovereigns in Israel during the earlier part of Asa’s reign 1 Kings 15:25-33, would naturally prevent disturbance on the part of the northern kingdom. The tender age of Asa himself would be a bar to warlike enterprises on the part of Judah.