Distinguish three age-counting systems
Traditional Korean counting age treats a newborn as one and adds a year every January 1. Calendar-year age starts at zero but also changes every January 1. International age starts at zero and changes on the person's birthday.
Calculate from years or a known international age
From years, traditional Korean counting age equals current year minus birth year plus one, while calendar-year age omits the plus one. Official international age equals calendar-year age after the birthday and one less before it. From a known international age, traditional age is one higher after the birthday or two higher before it.
Reproduce the published 1993 example
For a person born in 1993 and evaluated in 2019, traditional Korean counting age is 27. International age is 26 after the birthday or 25 before it, while calendar-year age is 26 throughout that year.
Use international age for current official contexts
South Korea standardized the international full-year age system for legal and social use from June 28, 2023. Some specific laws and programs can still define age differently, so follow the wording and reference date of the actual rule rather than using the traditional cultural result for eligibility.
Use a full date for exact eligibility
A year-only calculation depends on whether the birthday has occurred and cannot resolve a date-specific boundary by itself. For contracts, benefits, school, military service, alcohol or tobacco rules, medical records, or any official decision, use the required legal definition and exact birth and reference dates.