@hyeyoo Previously, the most widely used calculation method in Korea was the centuries-old "Korean age" system, in which a person turns one at birth and gains a year on 1 January. This means a baby born on 31 December will be two years old the next day.
A separate "counting age" system, that was also traditionally used in the country, considers a person zero at birth and adds a year on 1 January.
This means that, for example, as of 28 June 2023, a person born on 29 June 2003 is 19 under the international system, 20 under the "counting age" system and 21 under the "Korean age" system.
OK this is crazy, how confusing can things get? Haha