Portrait of the Yellow Emperor Huang Ti
(2698-2598 B.C.) 
The origin of the Chinese pharmacological book "The Herbal" (also known by various other names) has been lost in history and is now the subject of numerous myths and legends. The most colorful one is that in 2000 BC the Yellow Emperor himself ordered that the sum total of all the other medical works written or known previously before that time be put together into one volume. If true that would mean that at least parts of The Herbal, could go all the way back to 4500 BC (well before written Chinese script). Thus trying to establish the books exact age or even exactly when the hundreds of medicine herbs (including Cannabis) were first made mention of is difficult at best. Some say 4000 BC, others 2000 BC, but what is generally agreed upon is that by 500 BC medical Cannabis was solidly well establish in Chinese medicine. 
Part of a page from the Herbal 
 Picture of the Yellow Emperor courtesy of The Herbal Tradition http://www.planetherbs.com/articles/HerbHist.html The Herbal Tradition http://www.planetherbs.com
 For those interested in the subject the best translation of the Chinese Herbal can be found in: (Royal Asiatic Society) --- North China Branch, journal BRETSCHNEIDER, E.: Botanicum Sinicum, Materia Medica of the Ancient Chinese, Royal Asiatic Society of Great Britain and Ireland, North China Branch, journal, Shanghai, 1894-1895, Vol. 29.
 Defined as, the vast amount of experts in the field agree on this fact.