nine-men or nine-man crew

[fuadha]

Nine-men or nine-man crew
I'm with the second one.

[gabber]

Nine-men or nine-man crew
I'm with the second one.

You're right - an English attributive noun doesn't generally inflect for the plural, even when modified by a nuimber great than one :
A plan covering five years is a five-year plan, not a five-years plan (but check with a Brit on this)
A brush for the teeth is a toothbrush, not a teethbrush.
A tree bearing apples is an apple tree, not an apples tree
And a noun that is only part of a modifying phrase behaves the same way: a girl who's three years old is a three-year-old girl, not a three-years-old girl. (BTW, the grammatical head of this modifying phrase is "old," not "year.")

[konungursvia]

I agree completely, though I think we stick more closely to this Teutonic tradition than the Brits. I've seen them pluralize some of these things where we would leave them singular for sure.

[macanudo]

These are sometimes called noun + noun or nouns as adjectives. Basically,
it is always singular (no -s). There is an exception with fractions though:

He received a three-fourths share of the votes.

Also, it seems the hyphen (-) is used mostly in British English?

I just wrote an article about this and some other uses of nouns as adjectives for measurements. It can give you a good idea of the differences.