There are a number of diseases and disorders which can result in dementia, but Alzhemer's is the most common. It seems to be, basically, progressive brain rot. Its progress can be slow or quick. Dementia is not to be confused with the normal memory loss of aging: "Why did I just walk into this room?" "Where the heck did I put my iPhone?"
Alzheimer's is often given 7 stages.
I feel like I have been stuck at stage 2 most of my adult life.