I dreamed that grandpa was a little demented and didn't even remember me. What does this mean?

According to the duke of Zhou, not necessarily accurate:

Dream of the old man

A symbol of mental exhaustion. You will have this dream when your life is lifeless and you feel exhausted.

Dreaming that the old man died means that you will be reborn, which means that you want to start a new life in a new environment.

Dreaming that their ancestors disappeared after entering the house shows that some people think they can help themselves, but they can't.

Dreaming of grandparents

This dream means that you face a difficult problem in real life, but you must solve it intelligently and impulsively.

I'm sorry that I didn't "dream that Grandpa was a little demented", but dreams may not explain anything. When dreaming, one's normal physiological phenomena should not be taken too seriously.

I hope it helps you.