East Asian age reckoning

East Asian age reckoning is a concept and practice that originated in China and is used in East Asian cultures. Chinese culture, Japanese culture, Korean culture, Vietnamese culture, and others share this traditional way of counting a person's age. Newborns start at one year old, and each passing of a Lunar New Year, rather than the birthday, adds one year 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 two years old in their second year, three years old in their third, and so on. Since age is incremented on the new year rather than on a birthday, people may be 1 or 2 years older in Asian reckoning than in the Western system.

Today this system is commonly used in everyday life by Chinese in certain regions. The system is also widely used by Koreans, with the exception of the legal system and newspapers. In Eastern Outer 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 it is used for traditional fortune-telling or religion, and it is disappearing in daily life between people in the city.

Chinese
In either the traditional or modern age system, the word sui, 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 or modern age  or shisui.

Japanese
Japanese uses the word sai (歳 or 才) as a counter word for both the traditional and modern age system.

The traditional system of age reckoning, or kazoedoshi (数え年), was rendered obsolete by law in 1902 when Japan officially adopted the Western system,  known in Japanese as man nenrei (満年齢). However, the traditional system was still commonly used, so in 1950 another law was established to encourage people to use the Western system.

Today the traditional system is mainly used by the elderly. Elsewhere its use is limited to traditional ceremonies, divinations, and obituaries.

Korean
Koreans generally refer to their age in units called sal (살), using Korean numerals in ordinal form. Thus, a person is one sal during the first calendar year of life, and ten sal during the tenth calendar year.

The 100th-day anniversary of a baby is called baegil (백일), which literally means "a hundred days" in Korean, and is given a special celebration, marking the survival of what was once a period of high infant mortality. The first anniversary of birth named dol (돌) is likewise celebrated, and given even greater significance. Koreans celebrate their birthdays, even though every Korean gains one year on New Year's Day. Because the first year comes at birth and the second on the first day of the lunar New Year, a child born, for example, on December 29 (of the lunar calendar) will reach two years of age on Seollal (Korean New Year), when they are only days old in western reckoning.

In modern Korea the traditional system is most often used, mistakenly even when talking to non-Koreans. The international age system is referred to as "man-nai" (만나이) in which "man" (만) means "full" or "actual", and "nai" meaning "age". For example, man yeol sal means "full ten years", or "ten years old" in English. The Korean word dol means "years elapsed", identical to the English "years old", but is only used to refer to the first few birthdays. Cheotdol or simply dol refers to the first Western-equivalent birthday, dudol refers to the second, and so on.

The Korean Birthday Celebrations by the lunar calendar is called eumnyeok saeng-il (음력 생일, 陰曆生日) and yangnyeok saeng-il (양력 생일, 陽曆生日) is the birthday by Gregorian calendar.

For official government uses, documents, and legal procedures, the Western age system is used. Regulations regarding age limits on alcohol and tobacco use, as well as the age of consent, are all based on the Western system (man-nai).