How do you ask someone's "international age" (starting from zero, incrementing on their birthday; as opposed to Korean age) in Korean?
How to ask age regardless of calendar/time in womb:
TO GENERALLY YOUR AGE PEOPLE:
본인은 나이가 어떻게 되시죠?
For the person I'm looking at (you), the age...What has it become (risen to)?
TO ESTEEMED OLDER PEOPLE
본인은 연세가 어떻게 되세요?
(The translation is the same, and the sentence ending a tad more polite, maybe, with the word for age given honor).
너는 몇 살이야?
How many (years) old are you?
How to handle the request while excluding time in womb and infering :
There is a consideration for the fact that Koreans often use the lunar calendar and also count the time in the womb.
Asking age 만으로 will get you the solar calendar, more western age that is comparable to people from countries like mine, 미국.
My personal approach is to just get the age and follow-up with saying 만으로 어떻게 되세요? Often the listener will first respond with a qualifying question of whether or not we are to discuss this 만으로 or not making the need for the extra adverb less necessary.
Though it is not grammatically correct I have often heard adverbs added after the sentence to clarify the question, so this would be a much less awkward way to handle the question in one shot:
연세가 어떻게 되세요? 만으로...