Ya, Korean ages are different. I think it is becasue they use the lunar calendar so this might be true in other countries as well (like Japan).
The way it works is that you are one year old when you are born and then you gain a year on New Years day, not on your birthday. So if you are born in January you will be one year ahead of your regular age all year long except for those days between January first and your actual birthday; during that time you will be two years ahead of your actual age!!
Now, if you were born in December you would be one year old and then at the new year you would turn two so you would be two
years older than your standard age for most of the year.
Does that make sense?