My dog is 8yrs old and he will not come up the stairs to go outside. He mostly stays in his cage. What could be wrong with him.

It could be arthritis. He could be sick. It could be anything.

When a dog's behavior changes that dog needs to see a Vet.

I agree, It could be that he is painful or not feeling well. Take into consideration other things also, his appetite, his mood, attitude towards the family. And please, don't give him medications intended for people.
A visit to the vet may be in order.

Has he had any bad experiences of going outside or stairs because this could triggar a problem also this might need a trip to the vets as it could be a health problem