This topic should be "Difficult classics". Legend has it that this book was written by Qin Yueren during the Spring and Autumn Period and the Warring States Period. ~ ~ ~ However, this statement is not reliable. In fact, it was probably written in the Eastern Han Dynasty.
So,
From a recognized point of view, the oldest existing medical skill should actually be the Emperor's Internal Classic, which is basically confirmed to be written in the Western Han Dynasty.