Dementia patients become increasingly dependent on their caregivers, and as a caregiver, it isn't a great strategy to rely on giving the dementia patient advice and trying to convince them of something. Instead, you can model your role as trying to maximize the happiness of the patient's remaining lifespan, primarily through interventions that don't rely on the patient being agent-y. Some things that I have done for relatives with dementia that seem net positive:
Ultimately, communicating with and helping a dementia patient is extremely far removed from dealing with a rationalist. Instead, communication is primarily based on emotion and senses. In my experience, looking after dementia patients well, by providing the company, emotional support, and physical things that they need, slows down the progression of the disease as much as possible. Without this, dementia can progress extremely quickly, however in the presence of effective help, I believe dementia patients can experience net-positive lives for many years, as well as enrich the lives of their friends and relatives.
I agree with most of this advice. I probably couldn't do any better than that. But it seems unlikely to be the best that's possible. Dementia can probably be stopped in early stages if there's a way to persuade the patient to make larger lifestyle changes. It's frustrating that such persuasion is unusually hard.
Thank you so much for your thoughtful comment! Specific pieces of advice like these are exactly what I am looking for. One question: what is the rationale behind the brightly colored items?
I think I already have a good sense of what lifestyle changes can slow down aging and the progression of dementia and to what extent, but I don't know how to implement these changes. There is lots of advice here on how to change your life, which I appreciate, but not that much on how to change another person's life (please do correct me if I'm wrong), particularly if they are older and set in their ways or even suffering from dementia and find it difficult to learn new things while not quite comprehending the urgency of change. In this case, I couldn't even convince the primary caregiver (which is not, or at least should not be me) of the importance of diet and exercise.
How did/would you deal with these problems or any other problems in elder care in general? (Since my model of average rationalist told me that someone would bring up cryonics, I asked and they don't want that.)
As a side note, I feel like this topic does not receive enough attention on LessWrong, for instance it does not have a tag, unlike e.g. parenting.