Chrone

Chrone was the oldest planet in the universe. It was home to a rare real-space access point into the Time Vortex. The Vortex Code both contained the exact coordinates to Chrone and the means to control the access point, thus allowing the user to time travel. The Tenth Doctor followed the coordinates to Chrone where he and the Mark 9 Servitor containing the Code were confronted by the Stellion Core pirates who were after the Code. The Doctor repaired the Servitor and left it there with the pirates, who were not yet aware that he had secretly altered the Code so that it would only take them to the top three theme parks in that sector of the galaxy. (COMIC: The Vortex Code)

Behind the scenes
Planet One (TV: The Pandorica Opens) was also referred to as the oldest planet in the universe. Whether the two planets are one and the same is unstated in either story.