People in South Korea woke up on Wednesday to find that they were a year younger than they had been on Tuesday, and in some cases, up to two years younger.

This mass de-aging was down to a new law coming into effect, which scrapped a traditional age counting system, leaving South Koreans using the international method.

"We expect legal disputes, complaints and social confusion that have been caused over how to calculate ages will be greatly reduced," Minister of Government Legislation Lee Wan-kyu told a briefing on Monday.

How does South Korea's age-counting system work?

Under South Korea's traditional method of counting a person's age, an individual is already aged one at birth. Everyone's age then goes up on January 1, not on their birthday.

While the international system has been used for legal and medical documents since the early 1960s, the traditional system continued to be in use for daily life.

But the law, which was passed in December, has now brought that system to an end. And according to a government poll from September last year, 86% of South Koreans plan to switch to using the international system.

"I was about to turn 30 next year," Choi Hyun-ji, a 27-year-old office worker in Seoul, told Reuters, referring to the traditional system.

"Now I have some more time earned and I love it," they added.

Why does South Korea have different age-counting systems?

The traditional system had also previously been used in other countries in the region such as China, Japan and North Korea, but they all scrapped it decades ago.

It endured in South Korea in part due to the importance of "year age" to the Korean language since people generally use honorifics, such as "unni" — older sister — and "oppa" — older brother — instead of people's names.

"Age really matters," anthropologist Mo Hyun-joo told AFP. "It's hard to communicate with people without knowing their age."

With the traditional system, everyone in a school year was considered the same age and thus could talk without linguistic hierarchies.

A third age-counting system also exists in South Korea. It is used for calculating one's school year, when to join compulsory military service, and when one can purchase alcohol.

This system is similar to the traditional one, but individuals start at year zero. This system has not been affected by the new law and is set to remain in place.