One and a half year ago I had symptoms of dementia: not being able to remember words in every sentence I tried to speak, not being able to remember a serie of more than 5 numbers, and generally forgetting everything. It was due to heavy stress in my life, both at work and privately. Since the stress went down, the symptoms went away. So I agree with the previous post that dementia is very much stress related.
But with that said, I have seen my MIL becoming dement and she was early retired when that happened. She hadn't worked since the age of 50 and started the first symptoms between 60-65. She still used to be on a medical board to judge wrong treatment and she had to cancel that because she noticed she was becoming less sharp. During her FIRE, she did lots of things like knitting, sewing, painting wool with mushrooms, etc. But she was not very active and was also a heavy smoker. She did not have a good social life after relocating for FIRE and I think she missed that.
I don't know what caused her brain disfunction, but maybe the unhealthy lifestyle played a role. Her brother and sister who were equally unhealthy also got it.