How do I adress someone in Korean?
I've heard that 당신 is incorrect, but I have a few questions to ask about it.

1. What do I call a stranger who is my age?
2. What do I call a stranger who is older than me?
3. When is it appropriate to use 형/누나 outside of my family?
4. When can I use 당신?

Thanks for the answers.

Question

How do I adress someone in Korean?
I've heard that 당신 is incorrect, but I have a few questions to ask about it.

1. What do I call a stranger who is my age?
2. What do I call a stranger who is older than me?
3. When is it appropriate to use 형/누나 outside of my family?
4. When can I use 당신?

Thanks for the answers.

Answer

in most situations, you don't need to use 당신  
for example,
you want to know my name,then  just look at me and just say 성함이 어떻게 되십니까?(formal, polite) 이름이 뭐예요?(informal) 이름이 뭐야?(to children),In Korean,  a subject can be more easily omitted than in English. 
당신의(your) 이름은(name) 무엇입니까(what) is  literally changed with Enlish expression into Korean. it's unnatrual.

so,

1. 이름이 뭐예요?
2. 성함이 어떻게 되십니까? 
3. 형, 형님, //누나, 누님
4. when you fight with someone, 당신 이름이 뭐야(in angry voice) or when you mention a high postion being ,--오 , 주여 당신의 뜻이라면, 내가 따르겠습니다. oh, my Lord, if it is your intention, I will follow it.)

Answer

1.
2. older person -> young person : 너(not Polite expression), 당신(Polite expression)
3. friendly relation - man(boy) say to older man : 형, older woman : 누나
4. not friendly relation first meet.

sorry. my english skill bad.