22 But the Jews say again, This was said to Ahaz in reference to Hezekiah. Well, then, let us read the Scripture: Ask you a sign of the Lord your God, in the depth or in the height. And the sign certainly must be something astonishing. For the water from the rock was a sign, the sea divided, the sun turning back, and the like. But in what I am going to mention there is still more manifest refutation of the Jews. (I know that I am speaking at much length, and that my hearers are wearied: but bear with the fullness of my statements, because it is for Christ's sake these questions are moved, and they concern no ordinary matters.)
Now as Isaiah spoke this in the reign of Ahaz, and Ahaz reigned only sixteen years, and the prophecy was spoken to him within these years, the objection of the Jews is refuted by the fact that the succeeding king, Hezekiah, son of Ahaz, was twenty-five years old when he began to reign: for as the prophecy is confined within sixteen years, he must have been begotten of Ahaz full nine years before the prophecy. What need then was there to utter the prophecy concerning one who had been already begotten even before the reign of father Ahaz? For he said not, has conceived, but “the virgin shall conceive,” speaking as with foreknowledge.
Source: Catechetical Lectures (New Advent)