Why our age was not 9 month when we were born?

"In Korea, everyone is 1 from the time they are born. And everyone gets a year older on New Year’s day. So your Korean age is always either one or two years older than your Western age. And yes, a baby born on New Year’s Eve can be two years old the next day, although in practice people wait awhile before they start talking about somebody’s Korean age."

http://waegukin.com/korean-age-converter.html