Going to Korea ages you a year. Really.
The way Koreans count someones age differs from the European/North American system in two ways. Firstly, newborn babies are one year old, not zero. Secondly, everyone gets one year older at the same time, on Lunar New Year (when they eat a special kind of ddeok guk (rice cake soup) that symbolizes getting older), instead of on their birthdays. The first difference is more important, since it means that you must add one to your "Western age" to find your "Korean age," which is why going to Korea ages you a year. The combination of the two leads to some weird phenomena, like last year, when I became 24 (Korean age) before I became 23 (Western age); Lunar New Year fell in February, while my birthday is in May.
The effects on a Westerner coming to Korea are more than just superficial, however. Whether they realise it or not, everyone has a subconscious preconception of what it is to be a certain age, and feel a pressure to "act their age." When I first came to Korea, in 2001, I had to keep reminding myself to add one when someone asked me how old I was. Over time, it became natural, until I stopped thinking of myself as being 22, and started thinking of myself as being 23. Now, I'm 24, and it feels very unnatural to tell someone I'm 23, Western age, because I think of myself as 24, and I've noticed that it's carried over into my behaviour. I feel like I've matured two and a half years in the year and a half that I've been here. There are certainly other contributing factors, like being out of university and having a steady job teaching English, but I can't help but feel the age shift has something to do with it. I've lost a year of my youth. Who says age ain't nothing but a number?