I just wanted to mention a product that I found useful with our first dog as he aged. Max, our yellow Lab, was about five when he came to us and had calluses on his elbows. I tried different things, and he did grow some hair in those areas, but the calluses never really went away. He lived to be almost fourteen and in the last year or two of his life, the skin started breaking down on his elbows to the point where it almost looked like raw meat. I found a product online called "Canine Angels" that were padded elbow protectors and ordered some for Max and they were wonderful. It was kind of cute, it actually looked like he was wearing a pair of shorts, but the main thing is, it did the job! His elbows healed up and stayed healed up for the rest of his life. Our vet was amazed because he had seen Max's elbows before and said this was a problem with old dogs and there really wasn't much that could be done about it.
I am sure that are other companies that make similar products. I think elbow protectors can also be used sometimes to protect a surgical site or an injury, not just for senior dogs.