This is the most telling verse for the historical reference of this prophecy. Focusing on the virgin and the son moves our focus away from the essential prophecy, which is contained in verse 16. The Lord’s sign has two parts; a time frame, and a consequence.
The time frame involves the life of the son. Before this child is old enough to make the distinction between good and evil, an event will occur.
The event will be an invasion of Assyrian that will have dire effect on Judah, even though it was directed as the Syro-Israeli coalition.
"The fulfillment came about in successive stages. First, Tiglath-Pileser III (Pul) attacked Syria and Israel in 732 B.C. and took many Israelites captive to Assyria, especially those from the northern tribes. Secondly, in 730-727, Pul annexed the Transjordan area and departed large numbers of the Israelite tribes from that area to the far reaches of the Assyrian Empire. Third, in 726, Hoshea refused to pay Assyrian tribute, and Pul’s successor, Shalmaneser, retaliated by attacking Israel and besieging Samaria, which fell in 722 B.C. Thus, within a dozen years of Isaiah’’ prophecy, the alliance had completely failed, and three major groups of Israelites had been deported. Finally, large groups of the Israelites fled from Assyria to the remote areas northward and became the lost Ten Tribes of Israel. Apparently, within about fifty years of their leaving Assyria, they were scattered so widely that many of them no longer existed as a cohesive group. Thereby Isaiah’s prophecy to Ephraim was completely realized.
In the last part of the prophecy, Isaiah warns Judah that she must remain firm in her trust of the Lord or she will not be able to stand. Unfortunately, Ahaz did not heed this warning and relied instead upon the Assyrians for deliverance. The Assyrians did hinder the Syro-Israelite attack by destroying Syria and large parts of Israel, but since the Assyrians desired more territory and wealth, Judah found herself paying tribute to avoid war." (Ludlow, p. 141-2.)
The “sign”, therefore, refers to the destruction of Israel by Assyria, which will happen shortly (before a newborn male will have the opportunity to learn good from evil). This sign came within a time sufficient for Ahaz to see and comprehend the fulfillment of the prophecy, and therefore the sign.
What then of the virgin and the Messianic name? With the sense of the sign coming in the time for the destruction of Israel, the mother and the son are mechanisms in the sign, not the sign themselves. Reading “young woman” rather than “virgin” contextualizes the mother as a generic mother rather than a specific one. The name “God with us” is just as easily prescient of the fulfillment of the sign as it is a declaration of the mortal embodiment of the Savior.
In its historical context both mother and son are irrelevant. We need not search for any special woman nor ominously named child. They are meant to be generic, and are the medium for explaining a time period before the destruction of Israel, an event that Ahaz will live to witness. and the subject of the previous prophetic statements from Isaiah. Indeed, the verses following have strong echoes of the themes and language of those prophecies of the destruction of Israel.
Ludlow, however, disagrees with the above analysis, preferring to see the mother and son as real people in that time frame. After discussing the scholarly debate on the issue, he notes: “instead of the son of any woman fulfilling the promise, the son of one particular woman was designated. In the Hebrew, a definite article precedes the term translated as ”virgin“ or ”young woman,“ indicating that she is the virgin not just a virgin or any young women.” (Ludlow p. 144.)
However, he does agree that: “regardless of the precise identity of the woman and her child, it appears obvious that the circumstances of the child’s birth and the conditions surrounding his early life were so much an evidence of divine protection as to make proper his name, Immanuel, for God was truly with the people of Judah.” (Ludlow, p. 144.)