I knew about this, but I have one question, which age is used in legal aspects then? Like the age to get a driving license, the age to vote, the age making sexual relations with minors, normally under 18, illegal?

If you would use the Korean age, the age for those things would differ per person, right? That would not be just in my opinion.

Everyone's birthday is on New Years in Korea,  and you are born at age one. So if you are 17 you would be 19, and on January 1 next year, 20. Sorry, I am not very good at explaining this, but if you look up a Korean age converter, it will make more sense.

Those calculations are really confusing. I read from somewhere that it's much better (and easier) to ask for the year of their birthday instead of their age. What do you think about this? ^^

well, it's not like I'm about to add something new now.. but it's a bit less confusing way to calculate one's 'Korean' age: simply add one more year to your international age at the end of the current year. say, you' ll be 18 on the 31.12.2014 (does not matter if your birthday is earlier or exactly on 31.12). 18+1=19 -> your 'Korean' age. ~^^

To answer your question Chris, for legal things the international age is used so that it is easier to integrate with other countries. For example, when foreigners visit and they want to consume alcohol.

Korean age is mostly used to just express how old you are in a casual everyday setting.