For me this answers should be accepted. Just flag it.

얼마나 한국어를 공부해 와셨어요?

I answered from mobile app. It only had my in the words. Please report yours to feedback. I thought maybe our is a choice, too.

Literally it's something along the lines of "how is our grandma's age becoming". 몇 살이에요/몇 살이야 is a way of saying "how old is..." as well but it's not very respectful. It's what you would say when talking about somebody who is younger than you and not significantly higher on the social ladder either. For older people use 연세가 아떻게 되다 (with whatever verb endings the situation warrants). There is also 나이가 어떻게 되다 (again conjugated of course) which is somewhere in the middle (나이 and 연세 both mean "age", only Sino-Korean 연세(年歲) is a more respectful than native Korean 나이).

Makes sense to me. Literal translations are not 1:1 between Korean and English, but in this case it's close enough to make sense for me.

If you're talking to someone older than you, you can't use 몇 살이에요? You say 연세가 어떻게 되세요. Same thing with name, instead of saying 이름, you use 성함.

원래는 "우리의"라고 해야 할 텐데 실제로는 사람들이 "우리, 저희, 너희" 뒤의 "-의"를 생략하는 것이 흔해요:

• 우리(의) 할머니
• 저희(의) 회사
• 너희(의) 엄마

구어적 표현 뿐만 아니라 공식적 화법에서도 자주 듣은 형상이에요. "나의 --> 내; 저의 --> 제; 너의 --> 네" 등과 똑같이 흔한 축약형으로 보면 되겠죠.

Strictly speaking I guess it should be "우리의" but in reality people usually leave out the "-의" after "우리, 저희, 너희":

• 우리(의) 할머니
• 저희(의) 회사
• 너희(의) 엄마

This is not just a colloquialism; you can hear it in formal speech as well. You could view it as a contraction form which is just as common as "나의 --> 내; 저의 --> 제; 너의 --> 네" etc.