Your age is one thing that is cross-cultural, or so you would think. If you are 25 years old in the United States, you are 25 years old in South Africa. This should be the same everywhere on earth. Your assumption would be true just about everywhere, except in Korea.
For most of us, we turn one year old 365 days after birth, and you remain that age for the next 12 months. Not so in Korea. There, you are one year old on the day you are born. The only people who get to stay that age for a full year are those who are born on January 1.
January 1 is a significant date because that is the day everyone’s age advances by one year, regardless of what day you were born. By way of example, consider a boy who is born on June 6, a girl born on September 1, and a set of twins born on December 31. The age of each child is 1 year at the moment of birth. All of them will turn 2 on the subsequent January 1, even though none of them were 1 year old for anything close to an entire year.