This! My grandmother adopted a dog late in her life. She walked 10km a day with that dog for nearly 20 years! (That dog was the oldest dog I've ever known). At 92 she was famous in my small village, she was in better shape than some of the 30 year olds!
Then the dog died. Instead of walking 10km per day, she lay on the couch staring at the ceiling. About 3 months later she started getting lost on her way to the supermarket. Fifth time she got lost we decided to put her in a home for demented people. We simply couldn't provide the care she needed any other way. Took a few more months and she stopped recognising us.
I think she outlived her dog by about 18 months, iirc.
She stopped walking, and then age came fast for her.