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International age counts from birth (starting at 0), while Korean age starts at 1 at birth and increases by 1 every January 1st. As of 2023, South Korea officially adopted the international age system for legal purposes.

To calculate age in months, find the difference between the current date and the birth date in months. If the current day is before the birth day, subtract one month from the total.

People born on February 29th have a leap year birthday that only occurs every 4 years. In non-leap years, their legal birthday is typically March 1st in most countries, though some jurisdictions use February 28th.

Vietnamese traditional age (tuổi mụ) is similar to Korean age: a baby is considered 1 year old at birth and gains a year each Lunar New Year (Tết). The system is still commonly used in everyday conversation alongside the Western age system.