The interesting thing about dementia is that it is not normally distributed whatsoever. Pick basically any characteristic imaginable - environmental, behavioral

The interesting thing about dementia is that it is not normally distributed whatsoever. Pick basically any characteristic imaginable - environmental, behavioral, or genetic, and you're going to find a difference, often very significant, between groups. Everything from your occupation, to your diet, to martial status, to hobbies, and much more have been shown to have significant relationships with dementia rates. The problem you obviously run into here is that a person's approach towards healthcare is itself a major behavioral characteristic and so seeing varying rates of dementia based on this characteristic alone would be very unsurprising.