So, in Korea when you are born you are 1 years old. I believe this originated with starting age at the time of conception, but I'm not too well educated on the specifics. In addition to this different way of counting years, everyone gains a year on January 1st. So. Say there is a child born on December 29th. When he is born he is one years old. Come the new year, he becomes 2 years old. So, a 3 day old baby is already 2 years old.
Things get even more complicated when you consider the importance of age in this country. Asking age is normal here, and should not be considered an insult. It's merely a way to determine how much respect they should give you when they speak, because the language is based on honorifics that depend on age and status. If you are not the same age as someone, you can not consider someone a "friend" (징구: chingu). That's not that you can't be friends with someone of a different age, but you can't use the world friend. You must use terminology like older brother (오빠: opa) or older sister (어니: oni) or younger brother or sister (note: those are the words that girls use to call their older brothers or sisters. There is a different word to use if you are a man, but I don't know them, since I'll obviously never need to use them). So, this hypothetical person who was born on December 29th can not be "friends" with someone who was born 4 days later on January 2nd because they are not the same age.
This system was also used in other Asian countries, but it seems as though it's fading out in every other country except Korea.