The adjective "old-fashioned" does not take singular or plural
because it is an adjective. Only nouns (or other words acting as
nouns) can be singular or plural.
For example: Old-fashioned lady is one lady who's old-fashioned;
old-fashioned ladies can be 2 or more ladies who are old-fashioned.
In both cases, the adjective is the same; only the noun changes
according to number.