Dementia manifests differently for everyone, but it tends to have a sliding scale of severity that increases with time.
For people who get diagnosed early and who still have some degree of cognitive function, there are tools that , although they can't reverse the condition, can slow the progression and make it more manageable to go through.
Remarkably, music is potentially a valuable resource. At the very least, the right song for the right individual can temporarily revive a person's faded personality.
Neuroplasticity is the keyword. In this field of research we can find information to help us prevent and delay dementia in the first place, and deal with it better when we must face it.
My only long-form note on nostr happens to be on this very topic: