You don't need to say anything. If you need to just say "you".

If your question is how do you greet a person you have never met before in the street, you can just say 'Hello', 'Hi', 'Good morning', 'Good afternoon', etc. You don't need to use a title like 'lady', 'girl', 'Mrs', etc. in this situation. You say "(about sixteen years old or twenty-six)" - are you really asking if we use 'lady' or 'girl' for women of different ages?

If she's obviously unmarried you could call her "Miss".

In the US we don't really call people by their genders when initialing meeting. If I was meeting a girl for the first time I would just say "Hey, my name is Matty. How are you today?" Now if I was meeting an older (than me) woman of +10 years, I would say "Hello Ma'am." But I am in the Army so I basically have to call women Ma'am and guys Sir.