Quote Originally Posted by Antonio1986 View Post
As I understood девяностый is an adjective. All these are adjectives and are treated as adjectives.
They are adjectives (or, actually, ordinal numbers) used as nouns. It is a result of cutting off corresponding noun: a phrase "девяностые годы" turns to just "девяностые" for convenience. So such (adjectives) are used as nouns (but are declined still as adjectives).

In English it is the same: "Roaring twenties" etc.