[Grammar] Different kinds of adjectives?

[itecompro]

Hi all.

I'll immediately go through my question:
We say "He is a good boy" and we also say "He is good", but how come we say "He is 20 years old" but "He is a 20-year-old boy"? Why can't we say "He is a 20 years old boy"? Are these adjectives different kinds?

 

[SoothingDave]

When we use a measurement as an adjective, we switch to the singular version of the unit of measurement.

This ladder is 6 feet long. It is a 6-foot ladder.

The drive is 25 miles. It is a 25-mile drive.