International Age
Overview
Completed-years reckoning counts a person as zero at birth and increments on each birthday: age = current year − birth year, less one if the birthday has not yet passed this year. Someone born 15 June 1990, evaluated on 20 August 2026, is 36; the same person on 1 March 2026 would be 35. Nearly every legal system uses this convention, and South Korea aligned with it in June 2023, when the Age Counting Act made international age the default for administrative and civil purposes and retired the two traditional systems from official use. Two statutory exceptions survive: conscription eligibility and school entry are still determined by year of birth. February 29 birthdays are handled by jurisdiction-specific rules, with most treating 1 March as the legal anniversary in common years.
Variables
| Symbol | Name | Unit | Description |
|---|---|---|---|
| $Y_{now}$ | Current year | year | The calendar year of the reference date. |
| $Y_{birth}$ | Birth year | year | The calendar year of birth. |
| $Age$ | International age | years | Completed years lived, incremented on the birthday. |
Completed Years, Not Calendar Years
International age counts birthdays that have actually occurred:
$$Age = Y_{now} - Y_{birth} - (\text{before birthday} \;?\; 1 : 0)$$
The year difference gives a provisional figure; one year is subtracted whenever the reference date precedes the birthday within the current year. A person born on 15 September 1990, evaluated on 1 March 2026, has a year difference of 36 and a birthday still ahead, so the age is 35.
The Correction Term
The bracketed term is the whole of the difference between international age and Korea's year age. It is true for roughly half of any cohort on any given date — precisely those whose birthday falls later in the year than the reference date.
Legal Standing in Korea
Since June 2023 international age has been the default in Korean law and administrative practice. Statutes, contracts and medical records that formerly relied on counting age now use this expression unless a specific provision states otherwise, which removed a long-standing ambiguity in documents that simply said "age".
Derivation & History
The convention follows from treating age as elapsed duration rather than as calendar membership. Under a duration model the value increments at the completion of each full revolution since birth, which is by definition the birthday. Because calendar arithmetic gives the year difference cheaply, implementations compute that difference first and apply the birthday correction rather than working in elapsed days, which avoids leap-year and time-zone complications entirely.
Worked Examples
Born 15 September 1990, evaluated 1 March 2026
- Year difference: 2026 − 1990 = 36
- Compare month and day: 1 March precedes 15 September, so the birthday has not occurred
- Subtract one: 36 − 1
Result: 35 years
Edge Cases & Limitations
29 February births: a person born on a leap day has no birthday in common years. Korean practice treats 1 March as the increment date, while some jurisdictions use 28 February; the two differ for exactly one day each year.
The reference date's time zone matters: a birthday that has arrived in Seoul may not have arrived elsewhere, so cross-border systems fix a time zone before comparing.
Same-day births: on the birthday itself the correction term is false and the plain year difference applies.
Ages below one: the expression yields zero throughout the first year of life, which is why infant records use months rather than years.
Real-World Applications
International age governs eligibility thresholds in Korean law since the 2023 change — driving licences, medical consent, pension entitlement and insurance underwriting all read this value. It is also the age recorded in passports and immigration systems worldwide, so any Korean document destined for foreign use reports this figure rather than a counting age.