Koreans count their age different than other Western countries.
When a baby is born in Korea, she is already one year old because we count the pre-born time(9 months) as 1 year. And when a new year starts, she/he turns TWO years old. Therefore if a baby was born in December, she/he will be two years old in a month in the Korean age, but she is only a ONE month old baby. We call this full age system as 만. We use Korean age in our normal lives, but we use 만 나이 for papers.
I know it is weird age system we have, but they like this system because the age difference is very important to Koreans. If you are older than someone, the young one will use the honorific speech to you and they have to respect you more.
When I go back to Korea, I am older. When I come back to America, I am younger. So I like 만 나이 than 한국 나이 because I don’t want to be old!
만으로 나이(연세)가 어떻게 되세요? How old are you(is she) in the full age system?
저는 올해 한국 나이로 40이에요. I am 40 this year in the Korean age system.
한국 나이가 만 나이보다 많아요. The Korean age system is a lot older than the full age system.