One would wish that Isaiah could have been a bit more direct! Still, he is a prophet, and prophets are fond of oracular mysteries, and this is apparently one of those. Remember, he is here trying to assuage the fears of his king about the coalition formed against him by Israel and Aram. There is some woman (unnamed and unknown), who is pregnant, who will soon give birth to a son, whom she will name Emmanuel. And, the prophet continues, the boy will grow by means of a typical peasant child's diet of curds and honey. He will eat this tasty treat until the time he gains moral discernment. And when is that, you may ask? After all, some people never gain such discernment, do they? In your experience when do children gain this ability to know good and bad and find that choosing the good is a better course? Three years? Four years? It depends on the child, of course. Whenever that time is—and one can hardly know precisely—Isaiah says to Ahaz that when the child learns that important life lesson, Pekah and Remaliah, and their plan to destroy Judah, will find themselves kings over nothing and their design to conquer empty. Judah will be safe once again.
The sign of the child is a sign to Ahaz and Judah that YHWH is still watching over the fearful and fretful people of God's choice. And there is the meaning of the birth of the child we celebrate each year. He is a sign to us, fearful and fretful people, that God is with us still offering to us strength and love, assuring us that we are not finally alone no matter the size and force of the enemies we face. The story is not about a virgin birth; it is about a birth that demonstrates again that God is with us. And in that conviction, let us have a Merry Christmas, a secure Christmas, and may God help us to offer others such merriment and security, too.