Korea is a very hierarchy society. So if you want to call their name, you have to be older than them or have the same ages. You are younger than them, but you call their name, then you consider you are rude. haha..

oh you mean who can call somebody by their name... um... if you know him well (like since you were kids or students) or you are older than him, some cases you may use their name. For me, I call my friends from university by their name, but when we are in the same age. And you could tell when they call you first.