If you did a stand up comedy show for alzheimer's patients you could last 2 hours with just the one joke.
This joke is kind of insensitive, despite being sort of funny.
It's just a joke.  Alzheimer's patients don't know anything and it makes me laugh.
While it is true that people with Alzheimer forget things, it is kind of mean to make a joke like that. This joke may do well in a stand up, however it could offend someone who has dealt with a family member with this health problem.