From the article:

In South Korea, “international age” refers to the number of years since a person was born, and starts at zero – the same system used in most other countries.

But when asked their age in informal settings, most South Koreans will answer with their “Korean age,” which could be one or even two years older than their international age.

Under this system, which has its roots in China, babies are considered a year old on the day they’re born, with a year added every January 1.

[...]

Even with the new standardization, the old systems will still be used in some circumstances, the government said on Wednesday.

[...]

Laws on age-restricted products like alcohol or tobacco will also be based on the year someone is born, regardless of month. This means two people born in January and December 1990 are judged to be the same age.