“Unlike the international norm that starts with the age of zero at birth and then adds a year on birthdays, under the Korean system, a newborn baby's age is counted starting from one rather than zero, and everyone gets a year older all together on New Year's Day. The third system adds a year to a person's day on the first day of the New Year.”
https://www.koreatimes.co.kr/www/nation/2022/12/113_341383.html
