Few languages cluster consonants the way Indo-European languages do. Even fewer use final consonants to convey important morphological information. Arabic does have many ending inflections, but they differ from those seen in English. I think we're the world champions at using final consonants to convey dense amounts of information.
The person likely forgot year was supposed to be a plural noun, and / or that plural nouns are nearly always made plural by a final s (sometimes es). And if the person is an advanced student of English, you're likely right that his or her ear said "this sounds like something they say" and went ahead with it by analogy with "a twenty-year-old."
Or it could be a hypercorrection resulting from our Teutonic tendency to use singular nouns as modifiers for other nouns: a book-case, not a books-case; a recipe-book, not a recipes-book. It's odd if you don't know a Germanic language.
Student or Learner