Matthew in his gospel (1:22-23) translates it that way when citing the verse, but Matthew is either taking liberties with the Greek Septuagint (parthenos), being extremely creative with his messianic proof texting, or both. The Hebrew word, as all biblical students should know, is almah, meaning young woman, corresponding to alem, young man. If 'virgin' had been intended, there was a perfectly good word available, betulah. Isaiah doesn't use it.
Then there's the context of Isaiah 7. A prophecy? Well yes, but a very short term one. Reading on from v.14 through to v.17 the context makes it crystal clear that there's nothing here about a coming messiah. Interpretation of scripture in the first century was apparently a seat of the pants affair which most of us would scarcely recognise, whether for the early Christians or the sectarians of Qumran, so we can probably cut them some slack. But what excuse do the latter-day translators of the English Standard Version and the New International Version have?
Even the Catholic New Jerusalem Bible reads "young woman."
Digression: So does the NRSV, Revised English Bible, Good News Bible, New English Bible, Inclusive Bible, Moffatt, JPS. The Jerusalem Bible reads 'maiden.' Virgin is retained in the NAB, NASB, NLT, TNIV, CEV and Christian Community Bible.The passage in Matthew is legitimately rendered as 'virgin,' but any modern translation that reverts to 'virgin' in Isaiah is deliberately playing fast and loose with the Hebrew text. The ESV doesn't appear to translate almah at all, it just provides a face-saving apologetic substitution. Why pick on the ESV? Well, it's a particularly blatant example. The ESV is a revision of the 1952 Revised Standard Version, presumably updating and improving that text. Yet the RSV also reads "young woman."
It seems to be another example of "direct rectilinear messianic prophecy", cleaning up the older texts so the newer texts seem more accurate. If Isaiah won't say what we need him to say, why we'll change what he says, then stick out our chins and claim we're providing a more faithful translation!
It's comforting to find that there are committed Christian scholars who are willing to blow the whistle on this sleight of hand. Mention of Thom Stark's The Human Faces of God is relevant here. He calls this kind of thing "a conspicuous hermeneutic of convenience." Some time next week, all going well, there'll be a short review posted here, but for now the bottom line is, you really shouldn't have to perform a DIY lobotomy to be a good Christian (at least a non-fundamentalist Christian.)
But if you claim to provide an accurate translation, surely you do have to actually translate accurately.