Why Korean got 1 year old just right after birth?

Let's say Korean baby is born in 31 Dec 2017,
Next day(1st Jan 2018), s/he will be 2 years old even just 2 days passed!

What is the logic behind Korean age?

As far as I know, Korean think a embryo also as a human.
so that's why Korean baby got 1 year old after birth.

Therefore, traditionally Korean celebrate 100 days for baby(백일 or 백일잔치)
because 100 days is roughly 1 year from fertilization.

In short, Korean age contains respecting human being.
It's brief info so you can find further details from other sources.

Now, I will explain how to calculate Korean age

First, ignore your birthday is passed or not
Second, Keep in mind that Years are only needed for calculation Korean age

Formula is "Current Year - Year of your birth + 1 = Korean age(한국 나이)"

Ex) Suppose Today is 4th June 2018

- Case 1 b day : Feb 1989 -> To you, 29 / But, Korean age 2018 - 1989 + 1 = 30
- Case 2 b day : Sep 1994 -> To you, 23 / But, Korean age 2018 - 1994 + 1 = 25

I hope this will help you understand Korean culture :D