Is my dog sick/injured or just old?

I have a 10 year-old, male papillon. For the most part, he's really healthy and pretty much still acts like a puppy. He loves to run, jump, and play with his toys. Starting rather suddenly in the last few days, he's been less active and has had trouble jumping onto the couch or my bed (He's stubborn though, and keeps trying even when I go to pick him up.). If he jumps successfully, he sits there panting for a few minutes. Sometimes, he yips when people touch his ribs, abdomen, or lower back. I've checked him for injuries, but have found none. I have also felt his abdomen, and it does not feel rigid or bloated. He tenses if I get close to his hips. He's been drinking normally, has had normal bowel movements, no trouble urinating, and has not been vomiting. He has been eating slower, though (But he always finishes his bowl.). He still plays with his toys and runs around, but only if I instigate the playtime. Otherwise, he's rather lethargic. Could he be sick, or is it more likely old age?