Some aboriginal art is thought to be so old that it might depict megafauna. Here is a picture taken at the Arnhem Plateau in the Northern Territory that is thought to depict a Genyornis, a 2 metre tall flightless bird that went extinct around 40-55000 years ago.

Some cave paintings are over 40,000 years old, such as the ones in the cave of El Castillo (which is open to visitors). Here is an example of what's inside. Very cool!

If you're interested in old paintings, you might look into the Chauvet Cave in France. Some say these are the oldest figurative paintings in the world, though as I understand it there is some dispute about which cave paintings precisely are the oldest.

The German filmmaker Werner Herzog made an excellent documentary, Cave of Forgotten Dreams, about Chauvet cave. It's more philosophical than strictly art historical, but, like all his films, well worth checking out.

To piggyback, how about the oldest painting in a frame?

If I could add another question, What's the oldest painting on a man made canvas(animal skin, cloth, walls, pottery, etc.) that still exists?