Scholars agree that the Hebrew word almah (‎הָעַלְמָ֗ה), which the KJV translates as “virgin,” would be better rendered “young woman” or, perhaps, “single young woman.” It conveys a sense of sexual maturity but not of sexual abstinence. See Brevard S. Childs, Isaiah (Westminster John Knox Press, 2001), 65-66. The Hebrew Bible uses a different word, bethulah (‎הָעַלְמָ֗ה) to denote virginity, as in “And Ammon was so vexed, that he fell sick for his sister Tamar; for she was a virgin” (2 Sam 13:2). The sexual (or, rather, not-sexual) connotation of the term traces back to Matthew, who quotes the passage from Isaiah in Matthew 1:23, using the term parthenos (παρθενος). which does connote virginity in the modern sense of sexual abstinence.