It probably happens to everyone and certainly does here. One go along a path that is little changed for years and then something happens to change things - like someone stops working, you move, or someone close dies. I guess we all have to accept that change happens. It is part of life. When it happens it comes as a shock, but one has to move on.
In my case I had a stroke 2.5 years ago. At the time I expected to make a full recovery: 2.5 years later I still drive and walk and my intellect is still fine but my voice is very poor, I tire very easily with almost any physical or mental effort, and life is still a fog. Oh how I miss being my old self.