In South Korea, which generally became one birth, The reason is that before birth mother's womb is the period was 10 months. Isn't a very scientific? Age calculation in South Korea (current year - birth year +1)

I do believe that their age counts starts at 1 when babies are born. So instead of the normal age like me is 22 which is in the year 1988, in korean age, I would be 23 ^__^. So for Seohyun, she's 20 in korean age but in actuality she's 19. So it's not add 2, I think it's add 1 year. I think.. correct me if i am wrong on this..