What's wrong with my dog?

I have a 12 year old sheepdog mix. Lately he's been skipping going outside, has been very sedate, and his coat is becoming thin and scratchy on his back. He has a number of fatty tumors (have been checked by the vet). He's having a hard time getting up, barks at people he knows (like he doesn't recognize them), and barks for no reason. He's also gained quite a bit of weight. Are these just regular signs of aging or is there something wrong with him?