It depends on your degree of proximity. I've known of people as close to as my father-in-law who in his 90's developed mental issues (I don't think he was diagnosed) or his long-time neighbor who's Alzheimer's changed her from a vibrant woman into a shell of a human being. I personally experience some of that. But no immediate family member has succumbed to it.
Here's an analogy. I know you'll hate it. Try no to get too crabby.
I could get ALS and lose all my muscle control. But I don't think about that. I just try to keep myself in reasonably good physical shape. ALS will strike regardless of the condition of my body, but I benefit from the effort I put in, and it's far more likely that I'll never develop ALS, so it's smart to stay in shape.
Likewise, I could get Alzheimer's and lose all mental control. But I don't let that stop me from valuing and practicing peace, patience, kindness, and equanimity. I'm conditioning my mind, which will benefit me whether I develop Alzheimer's or not ... and it's more likely that I won't.
Let me bore you with a cautionary tale. You seem to believe you're not very likely to develop Alzheimer's. That exactly how my mother felt, well into her mid-70s. After all, she had never smoked, never been overweight, still exercised regularly every day, and had a BP reading that most 40 year olds wished they had. She kept her mind active, still practiced her shorthand skills, transcribing the dialog from TV shows she'd listen to and constantly finding new skills to learn. I gave her a computer and she learned to use a word processing program. She constantly read books, wrote letters, compiled local history, went to the movies with friends, traveled, socialized, and was involved in organizing various community projects. She could easily handle a 5-6 mile hike, and could drive for 6-hours on the Interstate with ease. Her mother had lived to be 84, and had been active until the day she died, with no signs of any mental impairments what-so-ever.
But despite of of that, and reaching her mid-70s and seemingly being physically and mentally healthy, for some reason, my mother suddenly began to exhibit signs of mental confusion. Small, but noticeable signs, that were very much out of character for her. I began urging her to see a doctor. She resisted, insisting she was fine. Her friends insisted she was fine. When she finally saw a doctor, he said she appeared fine.
As more time passed, I saw more signs. She reached to point of talking about "making time jumps" one day, and I took her to the ER, where I was told she had
cat-scratch disease, which had apparently caused her to experience
delirium.
A short time passed, then after she didn't answer her phone, I found her face down on the floor, semiconscious. At this point, a series test were finally run, but still no conclusive signs of Alzheimer's were found, nor were any evidence found of her having a stroke.
Months passed and she kept getting worse, withdrawing from any social life, physical activities, etc. All she wanted to do was watch TV all day, something she'd never done before. She kept insisting she was just getting older. Her friend insisted she was fine -- just let her relax and enjoy TV. She doesn't need to go to another doctor. Leave her alone.
Then, one day I went home to find her with her head down on the kitchen table, crying. She'd just watch the TV reports on the nightly news regarding the public announcement that President Reagan had been diagnosed with Alzheimer's disease earlier in the year. She kept saying "Now I realize what's wrong with me." And her next words were "There's no cure. No hope. I just wish I could just die now before I completely lose my mind. Is that wrong?"
She had practiced "peace, patience, kindness, and equanimity" all her life, and within her local community she was one of the few person who could diffuse virtually any disagreement with calmness, kindness, and wisdom. She could motive practically anyone or any group "to do the right thing" and had a gift for manipulating (in a caring way) people's opinions to her way of thinking.
But her ability to win over and influence people "to her side" was her undoing in regards to Alzheimer's. When doctors had interviewed her years earlier, it was she that was in control, and she could skillfully get out of any corners she might be boxed into. Every doctor had been convinced she was fine -- she was just a little forgetful -- nothing to worry about -- but in reality she knew how to hide the signs, and to put on a "good show". I was about the only person who had worried about her mental state. I was told countless times by doctors, friends, family -- she's fine -- you're overreacting. She could have been diagnosed years sooner, but I suppose it really wouldn't have mattered much...
Within a few months, my mom started having problem walking, then she reached the point of not being able to walk without using a cane. Within a few more months, she needed a wheelchair. And she gradually forgot how to do more and more things, and would forget who I was, what year it was, etc.
I managed to keep her in her home and cared for, by hiring people to stay with her. I suppose that gave her some comfort, but each day, she drift further away from reality, and would "time travel" into the past on some days and oddly enough, into the future on other days. She'd claim she had taken a ride on a steam train back to 1936 in the morning, and then talk about how "the spaceship from the other world would be coming soon to pick her up" later in the afternoon. Playing her piano was about the only skill she managed to hold onto until the end.
Personally, I do hope if I start showing the early signs of losing my mental faculties, that might hint I'm displaying the early signs of Alzheimer's, that I'll be able to admit to it, rather than trying to hide it. Not that it may matter very much... but perhaps there's something that can be done to give me enough time to wrap up at least a decent "bucket list" while I'm still able to do so.
, I decided research is in order and plan on taking the SAGE test. My Mom suffered from Dimentia, (not Alzheimer's) and was always concerned about her deteriorating memory. From that aspect, the good thing is that if you are worried about it, Alzheimer's, then that is not what is plaguing you. 
