pixiv's Oldest Drawing is a tag describing the very first drawing ever submitted that officially started the service.
The age of the submission is based off of the illustration ID. Single digit IDs tend to be test submissions for various websites. Unfortunately, the pixiv illustration ID of 1 is no longer available.
The first illustration ever on pixiv was submitted by Bacotu (Japanese: 馬骨).
Since Bacotu created pixiv, he also the first person ever to join pixiv and is the only member with an ID number that is lower than the official pixiv account. Bacotu's ID is 10 and the official pixiv account is 11.
The Legendary Illustration
Everything started with this kettle, which the illustration's ID is 20.
The second oldest illustration, with an ID of 21.
The third oldest illustration, with an ID of 22.