Earlier quoted context omitted.
There's no doubt that people decline cognitively in their old age. I don't think this study's saying that all cognitive decline is due to retirement. People get old and their bodies and brains deteriorate. To my mind the question is whether the rate of decline can be reduced by keeping mentally active.
Why would it be different from physical decline? If you dont keep exercising in your older years you will decline much faster.
Why would it be the same as physical decline? False analogy.