This answer to If there's nothing special about Pluto, why was it discovered so early? says:
In fact, often objects are discovered to be in pictures from long ago, such as Orcus, discovered in 2004, but pictures were taken of it in 1951.
This led me to wonder about solar system objects that were in fact discovered in very old data.
Answers mentioning notable discoveries that are old but may not necessarily be the "oldest" are also are welcome!