Yes, I have KOREAN AGE.
When I say that, NOBODY understands. Or if I calculate my age when I'm asked, they also get confused and ask me if I don't know my age.
Here's how Korean age works.
First of all, everyone is one year old the day they are born. Some people say we count fetus as life but I personally think this is not the reason. Secondly, everyone adds one year to their ages on "New Year's Day" altogether.
Then, 90% of Americans ask me "What? So everyone has same birthday?"
No, no, no.
Everyone has their own birthday, and we do celebrate our own birthday. But birthday is only when your International age gets a year older, which is not very important in Korean culture.
I know some of you are still confused. So here's my example:
I was born in 1992. The day I was born, I was one. After then, we counts only birth year. Therefore, I was two in 1993 and three in 1994.
Funny thing is a baby who is born in 31st of December will turn two the next day. This baby is actually two days old but two years old in Korean age. Because of this, Koreans use international for baby until about 36 months.
Koreans uses both Korean age and international age. Korean age is used for mostly social life. Drinking and smoking ages use Korean age, and introducing yourself to somebody. However, most of legal ages are International age. This is also confusing to Koreans so some people insist that we need to stop using Korean age.
Age is probably the most important thing in Korean culture. The word 'friend' applies only in same birth years. I can be a friend with some who was born in different year but I don't call them as my friend.
I just noticed that age in Korean culture is way more complicated than I thought.
So, If you are Korean, already be ready to explain this complication in 3-4 sentences. If you are American, I recommend you to research more about it. Or Studying in Korea as an exchange student is a great way to understand this weird age reckoning system.