Sure, mademoiselle usage is quickly dropping when applying to adults, regardless of the known marital status. Not to mention that nowadays, one fourth of the people living in couple in France are not married anyway.

Madame is a considered a mark of respect. Of course, I expect for someone having a romantic interest in you to quickly switch from madame to Mary (or whatever your first name is) without ever needing to use mademoiselle.

Yes it is common to call women madame if they look mature enough.
You would not call a 40 year old mademoiselle even if she looked younger.
I suppose it all depends on how you perceive the person. You could call a 25 year old Madame or mademoiselle, it is up to teh individual.