On the first question, yes, if you don't know the make-up of your audience, you should use the polite form. Even if you know you're speaking to people younger than you, it is a good idea to use polite language if you're addressing a multitude of people. By the way, there is also a distinction between "polite language" and "honorific language". The former concerns being polite to the listener, while the latter is for a third person mentioned in the talk, not the listener. For example: - 안녕하세요: polite to the listener. - 부모님 잘 계시니? not polite to the listener but honorific to 부모님 mentioned in it. In the second sentence, the style is casual but the word 부모님 ("parents" with honorific suffix "님") and 계시니 (계시다 which is an honorific version of 있다) are both honorific words. This is like saying "Your majesty" when you speak to the queen versus "Her majesty" when you refer to her while conversing with the prime minister. As for when you start to use non-polite style with someone, it depends on various factors. Children and teenagers usually talk among peers in casual language from the start, young adults do that somewhat later, and older adults much later. In some cases one of might get to say "Well, let's stop using the polite language between us", while in other cases the two might start with polite language, get to mix polite and casual, then eventually become casual in style.

It depends. Until highschool we all have same age so we don't use honorifics to classmates. But in university/college, usually people use honorifics until they become friends and say 'Let's talk in informal.'. So that's why most of korean people are asking ages to people when we first meet. In addition, the term 'Let's talk in informal/plain.' has a meaning of 'Let's be more familar.'.