On the age of Rehoboam at his accession, see 1 Kings 12:8 note. The 17 years of his reign must have been complete, or a little more than complete, if Abijam ascended the throne in the “eighteenth” year of Jeroboam 1 Kings 15:1.
Related Commentaries of 1 Kings 14:21
1 Kings 14:21
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 the LORD did choose out of all the tribes of Israel, to put his name there. And his mother's name was Naamah an Ammonitess.