Girl or woman? How do we make the distinction?

Why not ask the female human being in question? You'll get different answers and can use the address she likes.

If she was married , I'll call her woman, else call girl.

Never really thought about it. I generally refer to all unkown females regardless of age as "Excuse me, Miss" or " That lady over there"

How I refer to my female friends and companions depends a lot on the context of the conversation. If my wife and her female friends are taking a night to go to dinner on their own, we call it a "girls' night out," even though they are all in their 40s and 50s. In my classroom, I often refer to the 9 and 10 y.o. students as "young men" and "young ladies." Women I don't know who appear my age or older, I refer to as "Ma'am." Women clearly younger than myself, I refer to as "Miss."

Normally at 18 years old, they are called young woman. When 20+ woman, But in our country, that is not usually the case -- most of the time, when a woman become a wife, then she will be called a woman.

It depends. I may refer to a girl as a lady when she's 25 years old and I could call a lady "girl" when she's 35. Or the other way around. There is no limit for the age. I think some women prefer to be considered younger than they really are and they hate being called "lady" or "ma'am". But there are also some that get suspicious if you don't do that . I usually avoid using these words, I prefer to hint at this (when it comes to a woman's age, it's best to make her smile/blush ).

when i started paying taxes. somebody called me "miss" the other day and it kind of annoyed me. i'm almost 50 and married..i think "ma'am" fits in my case.

Yes, young lady is a nice way to express this. Young girls usually are not as developed as a woman. A woman is more voluptuous and just exudes a more womanly persona.