From my own research (ie googling), your best bet is to build up the brain, so if the decline starts, you have a stronger starting point. This means exercise[1], B12, folate, CDP-choline[2], turmeric[3], coconut oil (medium chain triglycerides) [4]. Also, I avoid paracetamol, there are some claims about it being linked to dementia.
There are some promising drug trials (aducanumab for example), but we'll have to wait and see.
[1]https://www.health.harvard.edu/blog/regular-exercise-changes...
[2]https://en.wikipedia.org/wiki/Citicoline