The traditional system, briefly
Under the traditional Korean age system, a person is considered age 1 at birth (rather than 0), and everyone's age increases by one on New Year's Day rather than on their individual birthday. This meant a baby born in December could be considered "age 2" just weeks after birth by traditional counting — a genuine source of confusion for visitors trying to do the math.
What's changed
Korea officially standardized to the international age system for most legal and administrative purposes in 2023, meaning birth certificates, contracts, and most official contexts now use the same age-counting convention used internationally. Traditional Korean age still persists informally in everyday conversation and some cultural contexts, but it's no longer the default for official purposes.
Why this still matters for visitors
You may still hear Koreans reference their "Korean age" in casual conversation, especially older generations, even though it's not the official standard anymore. If a birth year comes up in conversation and the stated age seems off by a year, this is almost certainly why — not a language mix-up.