[john70t]

There are a hundred different types of dementia:
1). Some are slow (#1 Alzheimer's).
A slow gradual drop over half a decade or more. Gradual fade out.

2). Some are fast (#2 Lewy Bodies).
A loss of memory combined with Parkinson's. Three years. Shakes. Everything. Roller coaster.

3). Some might possibly be medically treatable for a now.
If you don't ask, you don't know.

Forget your childhood angst.
Respect them and adapt.
Have patience.
Make good memories while you still both can.

Research 'sundowners syndrome'.