I don't think either causes or prevents dementia. They are both used by younger people without the same effects they have on the elderly/dementia patients.
The elderly have proven more susceptible to the side effects of benedryl. It causes a worsening in Glaucoma. It can increase BP which is often a problem in the elderly or lower BP making them more susceptible to falls. It can thicken secretions which can cause various problems in the elderly. Elderly men with prostate enlargement can have trouble urinating. I can also dry out the mucus membrane. I also makes you thirsty which causes problem with those on fluid restriction. Constipation is a side effect that is more dangerous in the elderly. Not sure of the reasons why but...Benadryl can cause memory loss in the elderly. A study conducted by University of Pittsburgh researchers followed elderly individuals given either benzodiazepine, such as Valium, Xanax and Ativan, or Benadryl for sleep. Benadryl was associated with an increase in cognitive decline. Many elderly individuals are on multiple medications. Benadryl can interact with some commonly prescribed drugs also used for sedation, pain and sleep, and also some antidepressants. It can also interfere with blood pressure lowering drugs, such as clonidine and propranolol (a beta blocker). Side effects of Benadryl that can affect anyone can be especially problematic for elderly individuals with multiple existing health problems and on a variety of medications.
Ativan is highly addictive even in the younger population and should not be used long term. Beyond this the elderly may not experience relief of anxiety with Ativan. It may have the opposite effect on the elderly, causing symptoms including mental/mood changes and increased anxiety. It is also not recommended for patients with depression... and many elderly dementia patients suffer from depression. It should not be used in patients with compromised respiratory systems... and many elderly dementia patients have compromised respiratory systems.
Maybe this helps with the understanding