I agree with D'Elle,
My Laddie died at almost 16. We adopted him when he was 18 months old and fed him one brand of high quality food along with random, dog safe people food. For most of his life he had a fish oil supplement and Chondroitin. He usually walked 3 times a day with 1 of those times involving a lot of ball play for up to 45 minutes when he was younger. He also went to the beach, hiking and to the barn with me. He went with us to family and friend events and was well socialized with people.
As he aged, he had arthritis and took Rimadyl for it. He lost most of his hearing around 14 and at 15 senility/confusion started to set it. His hind end also got very weak and I got him around with a Help Em Up harness.
My Jezebel is now 16, we adopted her when she was about 2. She has had the same food/routine as Laddie. She doesn't have arthritis, but has been diagnosed with a benign liver tumor. She takes a Denamarin supplement once a day for it. At 15 her hearing started to go and her eyesight at night is very poor. I use a lot of hand signals now and she follows them well. She has bouts of senility/confusion - sometimes I find her staring into space or facing a corner looking confused. I bring her back by touching her shoulder and she comes back to reality.
Overall, both of my dogs had a good diet, a variety of activities and we addressed medical issues quickly.