Compound Adjectives - Why They Should Be Plural

This is tricky grammar, but it is very common in English. So when you use this compound adjective, someone's weight 125 pound should stay singular. We can't make adjectives plural. Do not add S's anywhere in those words. 50 years old people or 50 year olds people like students aren't sure where to put the S. They know this is plural. I'm talking about many people and plural means add an S.