Demons were made up by people to answer the questions raised by some weird occurrence. Instead of looking at evidence and at the observations and coming to a logical conclusion, you can just say a demon did it. It is a bit like seeing a shooting star and claiming that it was superman.
As for the guy supposedly living for that long without food or water, unless he found a way to convert his urine into useable nurturance it is medically impossible. He would be as dry as a bone after a few weeks. Your cells need water and food to survive or they die.
I think a more reasonable conclusion is that he has been sneaking in food or someone has been giving him food. Also why would a demon" posses an old man and just sit around and not eat? Not very interesting life if you ask me.