Fully agree. Healthy lifestyle is certainly something that can prevent onset of dementia, to a substantial degree.
But there is an emerging possibility for early detection of some dementia-causing diseases that can potentially improve outcome _together_ with a healthy lifestyle. And there are very healthy-living people that develop dementia anyway - and these people are (implicitly) among the targets of the new drugs and tests. But these are in a minority compared with the rest of the population that develop dementia because of multiple causes, including poor lifestyle
There is a lot of reason in your story - there is a massive impact that poor circulation, obesity, diabetes and other cardiovascular diseases have on the brain and lead to dementia symptoms. We all have/had relatives that developed dementia - and it the decline is more obvious and quicker when compounded with other health factors
The issue is that "dementia" is in fact a very complex issue that can be caused by many compounding underlying causes. But a healthy lifestyle can in principle only help
Indeed, pure speculation about the age for a solution that make take decades to finish. The purpose of the speculation is to discuss the uselessness of the test at young age.
About the lifestyle recommendations, there's no point in making a test for dementia and get a recommendation to do the same to avoid dying at 60 YO from cardiovascular disease + diabetes combo.