...I'm pretty sure that writing fiction must also be good brain stimulation.
As it happens, my wife sent me to the doc as I can be forgetful. Doc's first question, What do you do? I write novels. Doc said, If you can do that, you're brain's fine. Lots of other reasons for forgetfulness. Next!
Actually, there are many things that can lead to memory issues, including but not limited to the following:
insufficient sleep
stress
breathing issues (oxygen deprivation)
At one point, I thought my father was in the early stages of dementia. What he actually had were pulmonary issues that prevented his brain from oxygenating properly. With supplemental oxygen, he was back to his old self in a week.
I've seen even high school students have memory lapses when they were under a lot of stress. Most extreme case: a student who forgot all about US history a day or so before the AP US History exam. He wanted to panic-study, but I kept saying, "Go home and take a nap!" Eventually, the advice penetrated, he went home, caught up on his sleep, and was fine the next day. Somewhat less extreme: I've had colleagues completely forget a conversation that we've had the day before.
In other words, there are many causes of memory loss aside from things like Alzheimer's. In much the same way that a headache doesn't necessarily indicate a brain tumor, memory loss doesn't necessarily indicate dementia. It's important not to jump t the worst case scenario without ruling out less serious possibilities (many of which are much more easily treatable).