Separately, regarding dementia, it is both less and more complicated than many commenters suggest.
From UCSF Neuroscience’s excellent Memory and Aging Center resources:
“Dementia is a general term for any disease that causes a change in memory and/or thinking skills that is severe enough to impair a person’s daily functioning… There are many different types of dementia… Most types of dementia cause a gradual worsening of symptoms over the course of years due to progressive damage to nerve cells in the brain caused by the underlying disease process…” [0]
Even more briefly: dementia is an umbrella term, many diseases can cause dementia, and those diseases may or may not be progressive.