As you know, everyone is already one your old from the time they are born. From then, everytime it's a new year (January 1st) you add one year. For example I was born on December 14th 1996. On January 1st 1997, I was not even 1 month old in international age, but in Korean age I was already 2 years old. An easy way to calculate your Korean age: (actual year - birth year) + 1.

Korean use three types of age-counting in their lives. Let’s assume a man was born on March,20th,2010, and today is March,19th,2015. The first of age-counting way is the international way ( used in almost all of the countries) As you know, his age is 14 (,because it’s the day before his birthday). The second way is the year-counting way. This way they just count the years after his birth. Then, they will say “He is 15 years old". (2015-2010) They use this way to tell his age easily . The third is the traditional way. In some countries of the East, people consider the age of a newborn baby as one year Because the baby was born after 10 months of fetal-life. And they think a man grows older on the first day of a year. Therefore the man above is 16 years old.(1+2015-2010) There is only one man, but he has three ages in Korea. Even if you heard the age of a Korean, you should know his real age after some close relations.