Isaih uses the word 'almah, which denotes a young, chaste woman, as you say, but – trouble is – the OT uses the word betuwlah in other places of the OT, which means specifically a virgin (from which virginity, or betuwlyim is derived.
Christian Bibles do not translate 'almah as young woman but instead use the word which specifically means a virgin, which Isaiah did not use.
I agree that the Hebrews more often used bethuwlah, but almah still legitimately means "virgin". However, if the word bethuwlah is taken in isolation, says Barnes, it does not necessarily mean that the woman was to be a virgin at the time of birth, although it is open to that reading. Here are Barnes' notes on "virgin" in Is. 7:14:
"Isaiah 7:14 [A virgin] This word properly means a girl, maiden, virgin, a young woman who is unmarried, and who is of marriageable age. The word `almaah, is derived from the verb `aalam, "to conceal, to hide, to cover." The word `elem, from the same verb, is applied to a "young man," in 1 Sam. 16:56; 20:22 . The word here translated a virgin, is applied to Rebekah (Gen 24:43), and to Miriam, the sister of Moses, Ex 2:8. It occurs in only seven places in the Old Testament. Besides those already mentioned, it is found in Ps 68:25; Song 1:3; 6:8; and Prov 30:19. In all these places, except, perhaps, in Proverbs, it is used in its obvious natural sense, to denote a young, unmarried female. In the Syriac, the word "alem," means to grow up, "juvenis factus est; juvenescere fecited." Hence, the derivatives are applied to youth; to young men; to young women-to those who "are growing up," and becoming youths."
"The etymology of the word requires us to suppose that it means one who is growing up to a marriageable state, or to the age of puberty. The word maiden, or virgin, expresses the correct idea. Hengstenberg contends, that it means one "in the unmarried state;" Gesenius, that it means simply the being of marriageable age, the age of puberty. The Hebrews usually employed the word bªtuwlaah, to denote a pure virgin (a word which the Syriac translation uses here); but the word here evidently denotes one who was "then" unmarried; and though its primary idea is that of one who is growing up, or in a marriageable state, yet the whole connection requires us to understand it of one who was "not then married," and who was, therefore, regarded and designated as a virgin. The Vulgate renders it 'virgo.' The Septuagint, hee parthenos, "a virgin" - a word which they use as a translation of the Hebrew bªtuwlaah in Ex 22:16-17; Lev 21:3,14; Deut 22:19,23,28; 32:25; Judg 19:24; 21:12; and in thirty-three other places (see Trommius' Concordance); of na`ªraah, a girl, in Gen 24:14,16,55; 34:3 (twice); 1 Kings 1:2; and of `almaah, only in Gen 24:43; and in Isa 7:14."
"The word, in the view of the Septuagint translators, therefore conveyed the proper idea of a virgin. The Chaldee uses substantially the same word as the Hebrew. The idea of a "virgin" is, therefore, the most obvious and natural idea in the use of this word. It does not, however, imply that the person spoken of should be a virgin "when the child" should be born; or that she should ever after be a virgin. It means simply that one who was "then" a virgin, but who was of marriageable age, should conceive, and bear a son. Whether she was "to be" a virgin "at the time" when the child was born, or was to remain such afterward, are inquiries which cannot be determined by a philological examination of the word. It is evident also, that the word is not opposed to "either" of these ideas. "Why" the name which is thus given to an unmarried woman was derived from the verb to "hide, to conceal," is not agreed among lexicographers. The more probable opinion is, that it was because to the time of marriage, the daughter was supposed to be hidden or concealed in the family of the parents; she was kept shut up, as it were, in the paternal dwelling. This idea is given by Jerome, who says, 'the name is given to a virgin because she is said to be hidden or secret; because she does not expose herself to the gaze of men, but is kept with great care under the custody of parents.' The sum of the inquiry here, into the meaning of the word translated "virgin," is, that it does not differ from that word as used by us. The expression means no more than that one who was then a virgin should have a son, and that this should be a sign to Ahaz." (emphasis added)