Why is Korean age different?
Age is calculated in an intersting way in Korea. A Korean person will say he or she is one or two years older than a non-Korean born in the same year. This is because Korean count the nine months that a baby is inside it's mother's womb, so the moment a baby is born it is already one year old. In addition, all Koreans gain another year at the passage of the new year, not on their birthday. Therefore, on babies' first New Year's Day, they will be two years old in Korea, even though it has been less than a year since they were born.
Another tradition in Korea concerning age is the use of the Chinese Zodiac. In Korea, the year in which a baby is born is linked with a Chinese Zodiac animal called "ddi"(띠). This began from an ancient Asian practice of representing each year of a twelve-year cycle with an animal. For example, the representative animal for 2008 is the mouse, so all people born in 2008 (or twelve years before or after) are mouse "ddi." For cultural reasons, Koreans are very interested in knowing each other's age, so asking someone's "ddi" are sometimes used as an indirect way of finding out.
-Write reference Korean Languge Education Center, Sogang University