Hello You could say that there are 3 types of speech Informal which is 반말 (한국 사람이야? = are you korean?) Formal which is 존댓말 (한국 사람이에요? = are you korean?) The most forma speechl is also called 존댓말 (한국 사람입니까? Are you korean? So 입니까 is even more polite than 이에요 although both of them are formal speech. So even if you use 이에요 to older people it is fine. Another difference is that 입니까 is only for questions whereas 이에요 could be used for statements i.e. 저는 영국 사람이에요 = i am english Therefore if you want to say the above sentence in the highest form, you need to say 저는 영국 사람입니다 Hope this helps^^

Hello~ "입니까?" is more formal way of speaking than "이에요?" "이에요" is also a polite way of speaking, but "입니까?" is the most polite form among all. Informal expressions are "이니?", "이야?" "이냐?" Hope this helps.