Quote:
is not pluralized is because it's not a noun; it's an adjective.

D'oh! You are correct. I didn't think before I responded.

Quote:
I've never heard of not pluralizing a noun that is preceded by a non-zero whole number


Well, besides the two cases I have already mentioned (certain lengths and measures), the other common case is when referring to livestock or game. I.e., "fifty head of cattle" or "today I shot fourteen dove." Again, the second example is most definitely a regionalism and is not considered proper -- just extremely common in certain areas. However, I believe "fifty head of cattle" is considered proper usage. (Webster, entry 4)