How do you describe age? Lots of people get this wrong!

17 days ago
One of the first things you learn in English is how to describe your age:

I am twenty five.

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?