Twenty years old was Ahaz when he began to reign: of the difficulty hence arising, See Poole on "2 Kings 18:2", to which it more properly belongs.
Related Commentaries of 2 Kings 16:2
2 Kings 16:2
2 Twenty years old was Ahaz when he began to reign, and reigned sixteen years in Jerusalem, and did not that which was right in the sight of the LORD his God, like David his father.