I think what Hailey explained is quite appropriate and there will be no better answer. Even though 오빠, 형, 누나, 언니 are quite friendly words which can be used to call someone who is basically older than you, I feel like to, as an adult working in a company, put some additional information on the usage of these words. These words are usually for the people who are very close to you in term of non-business relations. So, usually you cannot use them for your boss, senior or people who you don't really know well. Of course there are some exceptions which are shown below: 1) In a shop or restaurant of average quality, you will hear customers(regardless of male or female) calling young female staff as 언니 in order to make an order or to request her help. Here, 언니 doesn't mean anything respectful but it's only a word used instead of waitress or female assistant. 2) You will find middle-aged gentleman A calling younger man B with 형 after B's family name or sir name. It means that the gentleman A is saying the word, 형, in order to express his affection or friendliness toward the man B. 3) When a girl calls a man as 오빠, it is translated that he is slightly older than her, and if she thinks he is quite older than her she may call him as 아저씨. You could understand why Korean man want to be called as 오빠 by women who is, in fact, much younger than him. 4) There are other exceptions. I guess somebody is going to explain to you soon.

That's a great question. It depends on the birth month. I was born in December, so most of my friends born in 1984 are older than me. Several friends of mine are a few months younger than me. Their birth years are 1985, not 1984. I don't treat them like 동생 and they also don't call me 언니. We're friends like another friend. They are called 빠른 85년생, which means “Born in early 1985.”, they were usually born in January or in February or in March. Most Korean children usually enter elementary school at 8 years old in Korean age(*Korean age : In Korea, all babies are already one years old when they're born because they were in their respective mommy's womb for 10 months. :P). If some Korean children born in early 2000 start school in March, most of their friends' birth year would be 2001. So it's not true we Koreans call anyone a few older than us 오빠, 형, 언니, 누나.

simply speaking, the calling names are based on the year people were born in.

we do not call them 오빠 형 언니 누나 with a few months or a few days our standard is "year" kk