Ibn Ezra on Leviticus 21:13/אבן עזרא על ויקרא כ״א:י״ג

1IN HER VIRGINITY. There are nouns in Hebrew that never come in the singular. Compare, ne’urim (youth), zekunim (old age), alumim (youth), and betulim (virginity). There are other words which never come in the plural, i.e., taf (children), shekhem (shoulder), zahav (gold), and barzel (iron).