For before the boy knows enough to reject evil and choose good, the land of the two kings you dread will be laid waste.
16. Before the child shall know. Many have been led into a mistake by connecting this verse with the preceding one, as if it had been the same child that was mentioned. They suppose that it assigns the reason, and that the particle כי (ki) means for Bishops Lowth and Stock concur in rendering כי (ki) for, which indeed is its ordinary meaning.
Here, I. God, by the prophet, makes a gracious offer to Ahaz, to confirm the foregoing predictions, and his faith in them, by such sign or miracle as he should choose (Isa 7:10, Isa 7:11): Ask thee a sign of the Lord thy God; See here the divine faithfulness and veracity. God tells us nothing but what he is able and ready to prove.
Commenting on Isaiah 7:10-16
For before the child shall know to refuse the evil, and choose the good,.... This may be understood of Isaiah's child, Shearjashub, he had along with him, he was bid to take with him; and who therefore must be supposed to bear some part, or answer some end or other, in this prophecy; which it is very probable may be this, viz.