Why this error?

[fairgrace]

I need to explain why a student has made the following error. When writing about his brother he said "He is now 20 year old.", which I have corrected in "He is now 20 years old."
I am not sure why he made this error, but I thought one explanation might be that he wanted to use the composite adjective form 20-year-old, but didn't understand that the adjective should precede the noun and that hyphens are required. Or he simply didn't appreciate that years need to agreed with 20 (plural) and not to the adjective old (singular). Sorry, very confused and find the whole age business very difficult to explain!

[konungursvia]

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.

[fairgrace]

Thank you very much, konungursvia!