It is not as simple as subtracting one from the international age. An easy way you can think of Korean age is by counting how many years one has seen. By "seen", I mean counting as one year if you lived any part of the year. So if I was born on December 31st of 2015, I lived a partial day in 2015, and have also lived a few months of 2016, so I am two years old ("seen 2015 and 2016"), even though in international age I am zero year old (so there's a difference of two). One has to subtract two from his international age if he has not yet passed his birthday, and one if he has, to get Korean age. It is actually an Asian way of counting which existed in China and other Asian countries in the old days. And even in Korea, the official way (like ID's and other official documents) is to use the international system. And it makes sense in a way. If I was born on 12/31/2015, I have lived 2015 and 2016, which is two years. That is, if you think of "year" as a conceptual thing rather than a length of time, it makes perfect sense. It also doesn't have this weird concept of "zero year old".

If his birthday passed already then he's 29. If not, then 28.

29or28

29 years old :D Korean calculate age since we were in mom's womb so International age + 1 = Korean age

maybe 29