What Is Korean Age?
Korean age (세는 나이) counts a newborn as already 1 year old at birth, then adds a year to everyone's age at once on January 1st, regardless of individual birth dates. This differs from international age (used officially in South Korea since 2023), which starts at 0 and only advances on a person's own birthday.
Example
A baby born on December 31 is already considered age 2 in the Korean system on January 1 of the following year — just one day after birth — since the birth year counts as year 1 and the new calendar year adds another.