mykoreanstudy:

You start by speaking formally to almost everyone, because you don’t know them. The exception of course is if the person is obviously much younger than you (like a child), but this isn’t always the same for every person in every situation either. A lot of parents/family speak formally to very young children because they want the child to learn how to use formal language themselves. That being said, you can still pretty much always speak informally to a young child. Another exception to this rule is when there is another intercepting power dynamic, such as when the obviously younger person is a customer and the obviously older person is a cashier. But even this is based on situation. A cashier at a chain store will almost always speak formally to their customers, but a cashier at a non-chain may speak informally to some younger and/or familiar customers.

PS I heard that in Japanese you should always speak formally to a stranger, even if they are a child, maybe someone can confirm this, I thought it was funny. Another factoid about the difference between Japanese and Korean formalities: the most formal level (~습니다) is used much more by males than by females in Korea, while in Japan, speaking with high formality is considered to be more feminine and thus used much more by females.

It is somewhat common that people will state their ages promptly after meeting someone new. At this point, given it is not a formal gathering, it is OK to use 반말 to those who are younger/the same age as you. But even then some people will not immediately speak informally to those younger than them, because they are still an unfamiliar person. It depends on the speaker’s personality and on the situation in which they met said person. For example, in some schools/workplaces/institutions, rank is based not on age but on title or year of entrance. So in that situation, people may ask you that information instead and you should base your formality off of that. 

If someone is the same age as you, you can suggest that you speak informally to each other. If someone is older than you, you can tell them that it’s ok to speak informally to you. They may suggest that both of you speak informally to each other, in the case that the age difference between you isn’t that large. At that point it’s totally ok to speak to someone older than you informally, but you shouldn’t ever speak informally to someone older if they don’t tell you to do so. Continuing to speak formally to someone who asks you to speak informally can come across as a bit cold as well. So the basic rule of thumb is: the older person is in charge. If you are the younger person, follow whatever the older person asks you to do. If you are the older person, feel free to tell the younger person how you want them to speak to you. It could be a bit awkward to demand 반말 from someone who is a near stranger to you, or someone who you hardly ever see (although some people do this, generally very outgoing types who want to become closer to the other person quickly). 

If you are speaking to a group of people, you should speak formally if even one person in said group is higher than you in age/rank. You may also speak formally to a group even if they are all younger/below you, depending on the setting and on your personality. For example, it would be bizarre to give a speech to an audience in informal language, even if said audience were all younger/below you. When it comes to a teacher or professor in a class, some will choose to address their students formally and call their names with -씨, while other more laid-back or friendly teachers/professors may use informal language and refer to their students as -군/-양 or even -아/-야.

Also, I think I’ve mentioned this before, but Koreans consider speech to be more casual than writing, so in some cases they will speak to a person informally but write to them formally. You can see this a lot in how Koreans communicate with their parents: talk in 반말, text in 존댓말. So when writing to people in an unsure situation, it’s best to play it safe and write formally.

So in summary, there are five determining factors: age, rank, familiarity, personality, situation. It’s a hard thing to navigate. I’m still unsure a lot and even Koreans don’t know what they are doing sometimes lol.