Pluralization rule for “five-year-old children”, “20 pound note”, “10 mile run”

Question

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 1

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)

Answer 2

I think it all goes back to the -a suffix in Old English that marked plural adjectives, but which has since been lost. Perhaps if we'd had one a thousand years ago, we'd have called it A ten pounda note.

Switching from money to weight, some people would ask the grocer for Ten pound of apples. Presumably that's by association with the form we're talking about here, but I imagine most of us would use the plural in this case. So it's certainly not all cut-and-dried.

In short, it seems this is another case of 'language on the move' in ways we don't normally notice.