Learn English – How to find adjective form of a noun or noun form of an adjective

In your example, the sentence

These women will also look for younger men. And then you start seeing men in the streets fighting.

could be rephrased as:

These women will also look for younger men. And then you start seeing men fighting in the streets.

The word streets here is not being used as an 'adjectivized noun'. The writer does not want to stress that the men are engaged in 'street-fighting', he wants to say that a passer-by will start noticing fighting men now and then in the streets.

Had the author wanted to say what you thought he wanted, he would have written, for instance:

.. And then you start seeing men engaged in street fights.

A noun, when used in the adjective role, cannot assume the plural form. Some English nouns, however, look as if they are in the plural: news, athletics, although they are not really. Hence, we have a news reporter, an athletics coach - these may mislead a non-native speaker sometimes.

A changes to an if the following word begins with a vowel sound. It doesn't matter what part of speech the word is.

Silent letters are ignored. So you say "an hour" because the "o" is a vowel sound after the initial silent "h." Similarly: "an honor, a house."

Note that the "long U" sound, as in *uniform" is not considered a vowel for this purpose, so you'd say "a uniform" but "an ugly duckling."

"Y" and "W", while they typically have vowel sounds, are not considered vowels for this purpose, so you'd say "a yellow flower," "a wish", etc. (There's rare exceptions with words that begin with "Y" that have a "short I" sound, but all such words would be names or scientific words.)