Recent research has shown that THC can help to break down and prevent the buildup of a protein in the braun called amyloid beta, which is widely understood to expedite the onset of Alzheimer's. You can read the study about that here.
On the other hand, there is no hard evidence to suggest that CBD can stop, prevent or slow down the progression of dementia.
However, CBD has been shown to be effective in treating some of the most common behavioural symptoms of the disease - such as mood swings, agitation and aggression.
Recent studies also suggest that CBD has been shown to reduce or remove the impact of inflammation, oxygen buildup and brain cell decline, with inflammation in the brain having been shown to increase the negative impact of Alzheimer's disease.