South Korean babies born on December 31 become 2-year-olds the next day.

[soryang]

1. This is the reason

From the article:

There are also some who say the concept of “Korean age” encourages a fixation on age-based social standing in this seniority-based country. In South Korea, those born in the same year often treat each other as equals, while people must use honorific titles to address those born earlier, rather than directly using their names.

선배 (seonbae) Sino-Korean word from 先輩, from 先 (“prior”) + 輩 (“group”) meaning "senior"

https://en.wiktionary.org/wiki/%EC%84%A0%EB%B0%B0

후배 (hubae) Sino-Korean word from 後輩, from 後 (“after”) + 輩 (“generation”) meaning "junior"

https://en.wiktionary.org/wiki/%ED%9B%84%EB%B0%B0

Everyone is defined by whether they are senior or junior in Korean society. Classifying your age group determines who are your peers, who are senior, and who are junior to an individual in many social contexts. That determination will probably affect the form of speech (verb form, sentence ending, etc., not just the title) one uses to address others whether honorific, polite or familiar. It's embedded in the language.