If you are thinking of taking in a senior dog, try to get as much information as possible about the dog and its prior owner. Always be sure to get a health check by a veterinarian. It gives you a baseline to work with and will let you know if the dog needs any special medication or care. Sometimes the news is better than you think. Old does not have to mean sick, weak or infirm.
Many older dogs are very healthy well into their teens.
Important things to check are eyes, ears and teeth. Most dogs over ten years of age do not see as well as they did when they were younger. If they were humans, they would need reading glasses. Your veterinarian can tell you if your dog is getting cataracts serious enough to strongly limit his vision. Dogs do quite well with very limited sight, but you must be aware of dangers that they cannot see.