Firebase Denormalization and huge data requests

I've read all about NoSQL, that's why I'm asking, because I can't get how to certain things.
I get how to flatten data but I still don't get how to filter it from the Firebase side.

SELECT * FROM posts ORDER BY data DESC
This one is ok: ...url/posts").orderBy("-data")

SELECT * FROM postsByCat WHERE key = {cat} ORDER BY data DESC
This one is ok too: ...url/posts").child({cat}).orderBy("-data")

SELECT * FROM postsByCat WHERE key = {cat} AND age =>18 AND age <= 22 ORDER BY data DESC
Well, this one is much more complicated to me at the moment.
I know how to get the most of the data I want but when I have to filter the data in 2 or plus variables the things are much more complicated.
I need a logical explanation on how to do it with NoSQL. I really don't get how to do that.