
East Asian age reckoning

East Asian age reckoning refers to a few different systems of counting people's age that have been used in East Asia for thousands of years. People are born at the age of "one", i.e. the first year of their lifetime, and on New Year's Day (or Chinese New Year for the Chinese) one year is added to their age. That is, "age" is the number of calendar years in which they have lived. It uses an ordinal numeral instead of starting with "zero" years of age, which would be the cardinal age number. (However, under the cardinal age system the number is always positive, starting with one second and not measuring years until one has elapsed.) Since age is incremented at the beginning of the lunar or solar year, rather than on the anniversary of a birthday, people are one or two years older under Asian reckoning when compared to the international age system. Therefore, celebrating a birthday under East Asian age reckoning would not coincide with the incrementation of age from one numeral to the next as it would normally, so one's first birthday would be celebrated while they are in their second year.
The system originated in China and continues in limited use there along with Japan and Vietnam. It is common enough in South Korea to the extent where it may be known as "Korean age" ( or "counting age") and contrasted with "international age" (in China the term "real age" is used; in Japan, "full age"). South Koreans are also familiar with "year age", determined by the birth year and the current year. Eastern Mongolia has a different system for measuring a person's age, which is based on the number of lunar cycles that have passed since birth (for boys; girls are measured from conception).
China.
In either the traditional or modern age system the word "" (), meaning "years of age", is used for age counting. In Chinese usage, there are three different types of age reckoning systems:
Both the "traditional age" and the "modern age" have lost much of their popularity within China, and the "real age" is nowadays the most commonly used age reckoning system in China. Of the three, only "real age" (shísuì) is legally used when reporting age in an official document such as the passport, ID card or a bank paper. The "real age" is also the only legally admissible to start drinking alcohol. The "real age" is the current Chinese legal system of age reckoning.
When a child has survived one month of life (29 days, if using lunar month reckoning) a "mun yuet" () celebration can be observed, in which duck or chicken eggs dyed red are distributed to guests to signify fertility.
Korea.
Koreans who use the traditional system refer to their age in units called "sal" (살), using Korean numerals in ordinal form. Thus, a person is one "sal" ("han sal", 한살) during the first calendar year of life, and ten "sal" during the tenth calendar year. "Sal" is used for native Korean numerals, while "se" (세; 歲) is used for Sino-Korean. For example, "seumul-daseot sal" (스물다섯 살) and "i-sib-o se" (이십오 세; 二十五 歲) both mean 'twenty-five-year-old'. If the international system is used ("man-nai" (만나이)), then the age would be 만 스물다섯 살 ("man seumul-daseot sal").
The 100th-day anniversary of a baby is called "" (, ) 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. South Koreans celebrate their birthdays, even though every South Korean gains one 'sal' on New Year's Day. Because the first year comes at birth and the second on the first day of the New Year, children born, for example, on December 29 will reach two years of age on the New Year's Day, when they are only days old. Hence, everyone born on the same calendar year effectively has the same age and can easily be calculated by the formula: Age = (Current Year − Birth Year) + 1
In modern South Korea the traditional system is used alongside the international age system which is referred to as "man-nai" (만나이) in which "man" (만) means "full" or "actual", and "nai" (나이) meaning "age". For example, ' means "full ten years", or "ten years old" in English. The Korean word ' means "years elapsed", identical to the English "years old", but is only used to refer to the first few birthdays. ' or simply ' refers to the first Western-equivalent birthday, "" refers to the second, and so on.
The traditional system has not been used in modern North Korea since the 1980s. South Korea is now the only country that uses the East Asian age, which may consequently be referred to globally as "Korean age".
The Korean Birthday Celebrations by the lunar calendar is called ' (음력 생일, 陰曆生日) and ' (양력 생일, 陽曆生日) is the birthday by Gregorian calendar. In the past, most people used the lunar calendar, ', to tell their birthday more than the Gregorian calendar ' but nowadays Koreans, especially young generations, tend to use "" for telling their birthdates.
For official government uses, documents, and legal procedures, the international system is used. Regulations regarding age limits on beginning school, as well as the age of consent, are all based on this system ("man-nai"). The age qualifier for tobacco and alcohol use is actually similar to, but distinct from the East Asian reckoning system. A person is allowed tobacco and alcohol if it is after January 1 of the year one's age turns to 19 (real age). This is the "year age", which is basically (Korean age – 1), or when a person's Korean age is 20.
Japan.
The traditional Japanese system of age reckoning, or ' (, lit. "counted years"), which incremented one's age on New Year's Day, was rendered obsolete by law in 1902 when Japan officially adopted the modern age system, known in Japanese as "" (). However, the traditional system was still commonly used, so in 1950 another law was established to encourage people to use the modern age system.
Today the traditional system is used only by the elderly and in rural areas. Elsewhere its use is limited to traditional ceremonies, divinations, and obituaries.
Japanese uses the word "sai" ( or ) as a counter word for both the traditional and modern age system.
Because of the idea of "yakudoshi" or unlucky years, "kanreki" is a special occurrence for celebrating 60 years of life, meaning someone has returned to the same combination of zodiacal symbols that governed the year of their birth. 
Vietnam.
Having been influenced by Chinese culture, the ancient Vietnamese also used this system and, despite not being the official age on papers and in daily usages at the present, the East Asian age is still in limited use by adults, especially old people in rural areas. However, this age system is not really familiar to the younger generation. In Vietnam, it is called " ('her age'), "tuổi ta"" (literally "our age") or "tuổi âm" (Lunar-calendar age).
Eastern Mongolia.
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.