One of the first things you learn in English is how to describe your age:
Sometimes people say “have twenty five” but quickly learn the the correct way.
How can you describe someone *else* who is 25?
He is twenty five years old.
Ok, cool. But what is the difference when we say:
The twenty-five-year-old guy
Did you notice that “years old” changes to “year-old” in the second phrase?
Did you know about this difference?
Why do you think they are different?