East Asian age reckoning

Traditional East Asian age reckoning covers a group of related methods for reckoning human ages practiced in the East Asian cultural sphere, where age is the number of calendar years in which a person has been alive; it starts at 1 at birth and increases at each New Year. Ages calculated this way are always 1 or 2 years greater than ages that start with 0 at birth and increase at each birthday. Historical records from China, Japan, Korea, and Vietnam have usually been based on these methods, whose specific details have varied over time and by place. South Korea officially stopped using the older system on June 28, 2023. Informal use is still widespread in the Republic and People's Republic of China, North and South Korea, Singapore, and the overseas Chinese and Korean diasporas.

Chinese age reckoning, the first of these methods, originated from the belief in ancient Chinese astrology that one's fate is bound to the stars imagined to be in opposition to the planet Jupiter at the time of one's birth. The importance of this duodecennial cycle is also essential to fengshui geomancy but only survives in popular culture as the 12 animals of the Chinese zodiac, which—like the stars—change each Chinese New Year. In this system, one's age is not a calculation of the number of calendar years (年, nián) since birth but a count of the number of these Jovian stars (simplified Chinese: 岁; traditional Chinese: 歲; pinyin: suì) whose influence one has lived through. By the Song dynasty, this system—and the extra importance of the sixtieth birthday produced by its combination with the sexagenary cycle—had spread throughout the Sinosphere. Japan eliminated their version of this system as part of the Meiji Reforms. The Republic of China partially modernized the system during their own reforms, which were continued by the Communists after the Chinese Civil War. Modern Taiwan now has a mixed system, with very widespread use of traditional ages sometimes accommodated by the government. On the mainland, despite calculating age solely by birthdays for all official purposes, Standard Mandarin continues to exclusively use the word suì for talking about years of age; Japanese similarly uses its equivalent, sai.

Korean age reckoning began by using the Chinese system but changed to calculating ages using January 1st as the New Year with their adoption of the Gregorian calendar in 1896. In North Korea, the old system was eliminated entirely in the 1980s. In South Korea, medical and legal documents already utilised the international age system, where Korean age was still routinely used until 2023, when the government formally changed the legally recognised age system to bring it into line with international standards and resolve the confusion caused by multiple age systems. ^[1]

A third intermediate system has also been used by some South Korean laws. This "year age" is difference between one's birth year and the current year, equivalent to calculating ages using January 1 but starting at 0 instead of 1.