The first major problem is the context of this verse, the Christians interpret this verse to for tell something in the future.
1)The verse should not be in the future tense, it should read ' behold the young woman is with child and about to give birth to a son.' If the verse is in the present tense it would exclude Mary from being the young girl for she live much latter
2)The beginning of chapter seven says "In the reign of Ahaz son of Jotham son of Uzziah" .
This sets the time period
It also says in 7:10 "The L-rd spoke further to Ahaz' ask for a sign from the L-rd" and the sign he got is in 7:14 "Assuredly, my L-rd will give you(Ahaz) a sign of his own accord! Look, the young woman . . ." We see that the sign of a young woman giving birth was a sign for Ahaz
Another indication that it must have occurred shortly after Isaiah said it is because he used the Hebrew word ( twa) which indicates a prophecy which is fulfilled immediately, as in (Gen 15:4)(R.J.K)
(7:15)" By the time he learns to reject the bad and choose the good, people will be feeding curds and honey." Does G-d , omnipotent and all knowing, need to learn anything?(I.E)
Compare verses (7:16)"Before the lad knows to reject bad and choose good, the ground whose two kings you dread shall be abandoned." with (8:3-4)"I was intimate with the prophetess, and she conceived and bore a son . . .For before the boy learns to call `father' and mother the wealth of Damascus and spoils of Samaria . . . shall be carried off before the king of Assyria" There is a direct correlation between this two verse in the 'Before the lad knows" and the destruction of the foreign power.
The second area in which they make a mistake that they do not understand Hebrew,
The Hebrew word ( hmloo[h) indicates the age of a women and does not necessarily mean virgin. It was the common practice of the time for a man to marry a twelve year old woman. See (S.S. 6:8),(Gen 24:23),(Pr 30:19),(Ex 2:8), (Rt 2:),(Joel 1:8)
Even if it did mean a virgin shall conceive and bear a child, what is special about that. Every woman is a virgin who sometime in the future shall conceive and bear a child.(R.J.K)