adjectives - Pluralization rule for "five-year-old children", "20 pound note", "10 mile run"

Why are year, pound and mile in the singular form in the phrases below?

• five-year-old children


• 20 pound note


• 10 mile run

Is that because they're acting as adjectives, which are always invariable in English?

Is it incorrect to say...

• five-years-old children?


• 20 pounds note?


• 10 miles run?

Answer

Those are called compound and hyphenated compound adjectives. And adjectives don't have plural forms.

Additional examples

three-storey building (three-stories bulding)

four-wheel drive (four-wheels drive)

32-bit processor (32-bits processor)