90daykorean.com

xVx10 TIL Koreans all change age on the 1st of January. They still celebrate their birthday, but if someone asks you "how old are you" you are expected to answer as if you were born on January 1st. This also applies to legal majority.
gM7O8 TIL that in Korea, your age corresponds to the year, not when you came out of the womb. Everyone is 1 years old when they are born, and everyone age's up on the 1st of January. Making everyone's "birthday" effectively the 1st of January. This is separated from their "international age".