Some forms of dementia are the result of vascular disorders, and high blood pressure is more likely to exacerbate, or directly contribute to that kind of failure in older age.
Narrowing of the blood vessels supplying oxygen to the brain, caused by things like high blood pressure.
Also, strokes and mini-strokes.
This is all pretty new science though. Back when I went to school in the early 2000s instructors didn't even teach that the BBB permeability could change.
[0]: https://www.scirp.org/journal/PaperInformation.aspx?PaperID=...