My FIL just passed at 92. Now, while every dementia case is different, I could clearly imagine him having a finger-munching episode with his daughter, my wife, a year before he died. This Joe/Jill pic is now 8 months old.
My wife’s uncle had dementia and like you say, the wheels came off pretty quick. He was happy go lucky for about a year. Had no clue about what was going on around him or what happened 10 minutes ago. His condition went down hill in a matter of weeks.