Why do some cultures start counting age from 0 while others start with 1?

Under the Korean age system, their babies are considered to be one year old on the day they are born, and every January 1st a year is added to a person’s age, regardless of their actual birthdate. This is very interesting, a baby born on New Year’s Eve becomes two years old the next day.

Also, there is a second counting method, a mix of the international and Korean age systems which a baby is born at zero years and one year is added every January 1st.