Hi Brian and Peter.
Your subject here is one of interest to me. I was one of the contributors to Parkinson.s UK creative writing toolkit. The aim was to encourage engagement and improve the quality of life. I am a member of a couple of Parkinson's closed Facebook groups as well as using the creative corner here. Writing blogs and poetry is used to communicate and share. It provides a mechanism for engagement -reading other peoples work leads to a feeling of sharing and understanding. It allows the processing of difficult Parkinson's issues. None of this is necessarily holding off dementia but bear with me.
The cognitive tests usually show a correlation with how far the person went in education and this is a generalisation I have asked about this and the response I had was that achievement was in line with the ability to use mental strategies and problem solving. The thought being that those who learned strategies in earlier life have more tools to use their cognitive abilities.
Going a step farther have those who have cognitive issues the ability to relearn. The answer is maybe. If someone has forgotten the alphabet they may be able to relearn it by singing it. It is possible writing helps by increasing neural pathways giving more links and providing a stronger base for retaining word knowledge.
Another cognitive factor is tiredness and how "off" they are. Cognitive achievements are higher when we are at our best. This is important because if someone has cognitive difficulties if they can align tasks with feeling at their best they will achieve more. This could be doubly important because as soon as we believe something is beyond us it is.. So retaining confidence and self belief is also important.
I am not claiming we can hold off dementia but I do think we can perhaps maximise the effectiveness of our cognitive abilities. Creative writing has much to offer if we choose to use it but nothing is better than a well balanced active life.