Using honorifics?
When should you call someone Hyung/Noona? 형/누나

Should I still call a girl older than me by 1-3 weeks Noona?
should I call him Hyung when he's older by a month?

When can we drop the Hyung/Noona? how do I call someone at my age?
I've heard of "ssi" or should I just call them by their name? 씨

Question

Using honorifics?
When should you call someone Hyung/Noona?  형/누나

Should I still call a girl older than me by 1-3 weeks Noona? 
should I call him Hyung when he's older by a month?

When can we drop the Hyung/Noona? how do I call someone at my age?
I've heard of "ssi" or should I just call them by their name? 씨

Answer

In intimate relationship, If you are younger(Korean Age) than someone, you can say 형/누나.
In meeting formal  case or public case , you can say  *** 님 or *** 씨 to unknown someone .
If you know that someone is younger or similar age, you can say *** 씨.   
씨 is hard to use with old age.