Korean count age in different way.
When you're born, you're already 1 year old.
And then, plus 1 year on Jan, 1st every year.
Assuming your birthday is Dec.19, 1992.
You're 2 years old on Jan 1, 1993.
You're 20 years old this year 2011 in Korea.
So, when someone in Korea ask you how old you're, you can say "I'm 20."
However, if you want to tell your age like '19 this year going 20', you can say "만 19세(if your B-day passed)입니다" or "만 20세(if your B-day coming)입니다."
제 나이는 만 19세입니다. (My age is 19)
저는 만으로 19살입니다. (I'm 19 years old)