In contrast to the promise that Judah would not totally perish, Isaiah prophesied the fall of the northern kingdom, “the land that thou abhorrest,” which opposed Ahaz (2 Nephi 17:16). The two kings in the north at that time were put to death by the Assyrians.
The two nations of Ephraim and Syria would be destroyed by Assyria. Syria’s destruction came in 732 B.C. and Ephraim’s followed in 722 B.C. As noted by Elder Holland (see commentary for 2 Nephi 17:14 on page 83), the historic child of Isaiah’s time would be about 12 or 13 years old, the age set by Judaic law for moral responsibility.