Albert Barnes Commentary 1 Kings 14:21

"And Rehoboam the son of Solomon reigned in Judah. Rehoboam was forty and one years old when he began to reign, and he reigned seventeen years in Jerusalem, the city which Jehovah had chosen out of all the tribes of Israel, to put his name there: and his mother`s name was Naamah the Ammonitess." — 1 Kings 14:21 (ASV)

For information on the age of Rehoboam at his accession, see the note on 1 Kings 12:8. The seventeen years of his reign must have been complete, or slightly more, if Abijam ascended the throne in the “eighteenth” year of Jeroboam (1 Kings 15:1).