There is a bit of confusion as to what statins are and how they help with preventing dementia. The simplest definition of statins is that they are a particular group of drugs that lower cholesterol. (Remember, cholesterol is a significant part of several controllable risk factors that determine susceptibility to dementia.) In a similar way to some people taking an aspirin a day to prevent heart disease, there is a trend of people turning to statins to prevent dementia.
The problem with statins is that they are actual medical drugs that have all the warnings and contraindications that go with them. They are not like vitamins and supplements that can be subject to self-medication. While statins do work well for lowering cholesterol, not everyone who has high cholesterol should be taking statins. There is a rather specific profile (age, sex, cholesterol level, other medications, chronic illnesses, allergies, liver condition, etc.) that can truly benefit from the use of statins.
Certainly for people who have high cholesterol, and already at risk for developing dementia, going to a good doctor who is familiar with all the good and bad factors of statins and brain health will be a valid choice to consider. But perhaps the best way to go about lowering cholesterol is by preventing it. The side-effects of statins (especially from their misuse) are not worth the possible lowering of risk for dementia. The greatest danger that statins pose is that they can become an excuse for not correcting the habits that cause dangerous levels of cholesterol.