In South Korea, when a baby is born they are considered a year old. Come New Year's Day, they gain another year. This means a baby born in December would be considered to be two years old in just a few weeks
K-pop band BTS's Kim Tae-hyung aka V, born on 30 December 1995, is 28 years old (Korean age), 26 years old (the international age) or 27 years old (another Korean official age).
Why Koreans could soon become a year younger
https://www.bbc.co.uk/news/world-asia-61117434