I came to Korea being 24 (US age) so I got off of the plane and was 25 (Korean age). Then a few months later, in January I was already 26 (Korean age) before I even really turned 25, since my birthday is in February.
And age is of course a big thing here. You can be certain that when someone is introduced for the first time to a group, or when meeting new friends, people ask what year you were born. If affects your social status.