I have already addressed the differences and quirks of the Korean "age" system but it turns out that it's a little more complicated than I thought. They are one year old when they are born. The day of their birth is recognized as their "birthday" but their age doesn't ever change on that day. It changes with the start of the New Year. Traditionally Korea has followed the Lunar calendar, so the New Year starts sometimes in February. But with the westernization of the country, some families have moved over to the Julian calendar so they now age by one year on the first of January...but not all of them. Some of them change their age in February when it becomes the year of the Rooster. So, if you were born on December 30th, you would add a year to your age on the first of January, which would mean that by Western standards you are two days old and by Korean standards you are two years old. Sounds simple enough right?
My kids at school are all suddenly a year older and I can't keep track of how old they are "for real" and they keep asking me how old I am and I don't even know anymore. My Korean vocabulary is expanding but I still keep coming back to the same old word, "mua-li-yo" - I don't know.