When do Korean babies go from "new-born" (신생아) to "infant" (영아)?

ANSWER: At 4 weeks

A new-born baby (신생아 新生兒) is a “new-born baby” for the first 4 weeks of its life. After that, until it is 1-year old, it is an “infant” (영아 嬰兒). From 1 up to 6 years old, it is a “preschool child” (유아 乳兒). And from 6 to 12 years old, it is a “preadolescent child” (아동 兒童). From 12 to 19 years old, it is an "adolescent child" (소년 少年 or 청소년 靑少年). And from 20 to 64 years old, it is an “adult” (어른). From 65 years old until whenever, it is a “senior citizen” (노인).

The above is just my general interpretation of the information provided in the pages shown below, which are taken from the book shown below. Certain laws and regulations in Korea define some of the terms slightly differently, something that is explained in the book.

The guy who wrote the book is the same guy who wrote the THIS BOOK. As for my using "it" in my description, feel free to substitute your preferred pronouns.