Origen seeks to balance lexical nuances and logical conclusions when interpreting the "virgin" vs. "young woman" in Contra Celsum 1.34–35.
But if a Jew should ingeniously explain it away by saying that it is not written 'Behold a virgin' but, instead of that, 'behold a young woman', we should say to him that the word Aalma, which the Septuagint translated by 'parthenos' (virgin) and others by 'neanis' (young woman), also occurs, so they say, in Deuteronomy applied to a virgin.
However, lest we appear to depend on a Hebrew word to explain to people, who do not understand whether to accept it or not, that the prophet said that this man would be born of a virgin (concerning who's birth it was said 'God with us'), let us explain the affirmation from the passage itself. The Lord, according to the scripture, said to Ahaz: 'Ask for a sing from the Lord your God, either in the depth or in the height.' And then the sign that is given is this: 'Behold a virgin shall conceive and bear a son.' What sort of sign would it be if a young woman not a virgin bore a son? And which would be more appropriate as the mother of Emmanuel, that is 'God with us', a woman who had had intercourse with a man and conceived by female passion, or a woman who was still chaste and pure and a virgin? It is surely fitting that the latter should give birth to a child at whose birth it is said 'God with us'.