|Mother Mary, ever-Virgin, Pray for us!|
There's a few things to unpack here. First we have the issue of "the author of Matthew" (that would be St. Matthew, nothing tricky here unlike "who is buried in Pope John XXIII's tomb?", pace some modern biblical theologians), using a "different translation then (sic) the Hebrew one." Matthew, and most other New Testament authors, when quoting from the Old Testament, quotes from the Greek translation of the Old Testament books known as the Septuagint. The Sepuagint is an ancient translation with quite an interesting history, of which we will only provide a brief thumbnail sketch.
The ancient king of Egypt, Ptolemy II Philadelphus (287 - 247 BC), when constructing the famous library at Alexandria (the greatest collection of ancient wisdom in the world) decided that he needed a Greek language copy of the Jewish holy books. To this end, he sent to Jerusalem for a copy of the aforesaid holy books and for Jews learned in the Law and fluent in Greek who could translate the books into the lingua franca of the ancient world. The high priest in Jerusalem selected six men from each of the twelve tribes, a total of seventy-two men, and sent them off to Egypt. These seventy-two scholars created the translation now known as the Septuagint.
While the exact history of these translators has been lost to history (as has the library the translation was made for), the antiquity of the translation is certain. In fact, the Septuagint is older than the Massoretic manuscripts (the one's we have in Hebrew), making the Septuagint Greek more reliable than the later Hebrew manuscripts. It is for this reason, as well as for the overwhelming use of the Septuagint translation in the New Testament, that the Church has always regarded this as the definitive Old Testament translation. In other words, "the author of Matthew" wasn't using a "different translation then (sic) the Hebrew one," he was using a better one.
Second, it isn't "logical" that Mary wasn't a virgin even if the Hebrew manuscripts were better and even if they didn't predict that she would be a virgin. The Hebrew doesn't say she wasn't a virgin, therefore concluding that she mustn't have been one on the silence in the text is very, very illogical (indeed it is a logical fallacy).
Third, the passage from the New Testament you are referring to is Matthew 1:23, which quotes Isaiah 7:14
Behold a virgin shall be with child, and bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.The word (ἡ παρθένος) is clearly "virgin" in the Septuagint, but does the Hebrew also have the same meaning? Or, as you claim, does it simply mean "young woman of child bearing age?" The Hebrew word in question is 'almah, which, in the Old Testament, only ever carries the meaning of "virgin" (see Genesis 24:43; Exodus 2:8; Psalm 68:25; Proverbs 30:19; Song of Solomon 1:3; Song of Solomon 6:8). This consistent use of 'almah throughout the OT in addition to the ancient, and very accurate, Septuagint translation of 'almah as "virgin" provides conclusive evidence that both the Hebrew and the Greek versions of Isaiah foretell a virgin giving birth. Besides, how interesting is it to "behold a young woman of child bearing age shall be with child?" If that translation could hold true, one would wonder why we are "beholding" that mundane fact.