"친구에 나이가 어디 있어요?" - 친구에: 친구(friend) 에(in, for) => in(for) friendship; regarding friendship. - 나이가: 나이(age) + 가(subject marker). - 어디 있어요?: 어디(where) 있어요?(is) => where is it? So it is literally "Where is age in friendship?". "Where is ...?" in this case is a rhetorical question meaning "there can't be..". It means "왜 나이가 있어야 해요?", "나이는 필요없어요", "나이는 상관없어요", etc. So what it really means is "Age doesn't matter when it comes to friendship".