South Korea

Koreans are born a year older than us (at least).

An excerpt from the article 15 facts about South Korea

The age of a person born in Korea is counted from one, not from zero as in our country. This is because Koreans include the fetal period in life expectancy. Moreover, with the new year, all citizens automatically age one year, unlike the rest of the world, where we add 1 to our age on our birthday. If you are born on December 31st, you will be 2 years old the next day!