dementia fears and reality

Sometimes, while flicking quickly through an attractive title, I realise only after settling down to a good read, that I have read the book before. Is my brain shrinking with the passing of time? When did this begin? How can I check it is happening and the rate of advance if I cannot remember in the first place?

Even more worryingly, very fine novels reveal aspects of themselves I had not noticed formerly (or had forgotten?).

Comments on re-reading experiences should be either condemnatory or, preferably, reassuring.