East Asian age reckoning

East Asian age reckoning is a concept and practice that originated in China and is widely used by other cultures in East Asia. Newborns start at the age of one year, and at the New Year's Day one year is added to the person's age. In other words, the first year of life is counted as one instead of zero, so that a person is one year old in their first year, two years old in their second, and so on. Since age is incremented on the beginning of lichun, which is the first solar term of the 23 solar terms rather than on a birthday, people may be one or two years older in Asian reckoning than in the modern age system.

In China, the age changes on the lichun. The current age reckoning system in use in South Korea is based on the Gregorian Calendar, though originally Koreans also followed the lichun as the beginning of the year and also the date for change of age.

In Eastern Mongolia, age is traditionally determined based on the number of full moons since conception for girls, and the number of new moons since birth for boys.

In Japan, Vietnam, and South Korea, New Year's Day as the date for change of age is used for traditional fortune-telling or religion.

The idea of a universal birthday disappeared from all of the Sinosphere, China and Japan having switched over to the western age reckoning system.

In either the traditional or modern age system, the word sui (traditional Chinese: 歲; simplified Chinese: 岁; pinyin: suì), meaning "years of age", is used for age counting. When a person's age is given in a publication, it is often specified whether that is his or her traditional age (traditional Chinese: 虛歲; simplified Chinese: 虚岁; pinyin: xūsuì) based on the East Asian reckoning system, or modern age (traditional Chinese: 周歲; simplified Chinese: 周岁; pinyin: zhōusùi) or shisui (traditional Chinese: 實歲; simplified Chinese: 实岁; pinyin: shísùi) based on the Gregorian calendar.

...
Wikipedia