Charles Ellicott Commentary 2 Kings 16:2

"Twenty years old was Ahaz when he began to reign; and he reigned sixteen years in Jerusalem: and he did not that which was right in the eyes of Jehovah his God, like David his father." — 2 Kings 16:2 (ASV)

THE REIGN OF AHAZ.
(Compare to 2 Chronicles 28:0)

Twenty years old. —The number should probably be twenty -and- five, according to the Septuagint, Syriac, and Arabic of 2 Chronicles 28:1. Otherwise, Ahaz was begotten when his father was ten (or, eleven) years old—a thing perhaps not impossible in the East, where both sexes reach maturity earlier than among Western peoples.