Yes. It is referring to the "stone age." As in; those painting are so very old, straight out of the stone age.

Here's the passage if anyone wants to check it: https://ebooks.adelaide.edu.au/f/fielding/henry/joseph_andrews/book1.2.html It's definitely not about the stone age. However, it is a strange turn of phrase and I wondered if it was a playful reference to "epitaph" (ie. "what is written on a gravestone"). The best guess I can give is that it's a reference to the text written on King Arthur's mythical "sword in the stone". Old, poetic verse with a prediction. Lines that sound like verse from the Dark Ages. Does that make sense to you as well?

doesnt say the stone age? i think they are just trying to show the words used were old fashioned or dated