Korean has a totally different age system with other countries. It is known that the system is used among countries that use Chinese characters. Surprisingly, other countries like China, Vietnam, and Japan don't use the system anymore. Only Korea does. Koreans get one year as soon as they are born. When the year of one's birthday passes, they get one more year.
For example, Gary was born on July 8th, 1991. He is already 1 year old on at this point. On July 8th, 1992, Gary's 2nd birthday came. He is now 2 years old.
However, western age system is also used in Korea. Koreans sometimes use 만 O세(살). '만' indicates one's western age. So when Koreans use 만 O세 they are referring their western ages.
If Bobby is 9 years old (western age) then Koreans would say Bobby's age is 만 9세.
To calculate your Korean age, go to Download page to download a Korean age calculator.