I don't feel that Christians will develop dementia.
I'll just come right out and say it: Christians do not get dementia.
Recently, I've come to acknowledge that both of my grandparents are suffering from some level of dementia and they are probably winding down in the latter stage of their lives. I've had to ask myself, why are they suffering this dementia? They have always been churchgoers, for decades, they raised their kids in church, they've read their Bibles.
So I had to answer an honest question when I was confronted with a possible conflict.
When I look at it, and I look back on the years I've known my grandparents, I don't see any evidence they were born again. That doesn't mean I believe their souls are lost. I believe they had saving knowledge, but I would not call them Christians. A Christian is what I would call someone who genuinely loves reading the Bible, Bible study, and holy living. I don't recall ever reading the Bible with my grandparents. Instead, the one time I do recall talking about it, they treated me like I was a freak, a religious nut. And as far as holy living, the extent of their religion said "gluttony", eating too much was a sin. Which basically meant quietly or privately passing judgment on anyone who in their judgment "ate too much". The Rock eats 9,000 calories a day, and some people consider him to be the epitome of a physique. And never mind my grandmother was pretty big back when I was a kid.
I think it was my uncle who said back in the day, the 1950s, my grandparents actually kept an ashtray lying around, they were liberal enough to respect any possible guests who came in and wanted to light up a cig. Although they themselves were neither drinkers nor smokers. That's pretty cool because most people are hardcore one way or the other.
Now socially, my grandparents were big into cultural Christianity, specifically the Christian gospel music scene. This isn't really a force today, but years ago, the Gaithers, Ivan Parker, and other groups were quite popular in mainstream Christian, but the music scene was considerably different as well. Country western music WAS "pop" music. Popular music charts were dominated by country and crossover artists. I doubt if anyone my age really knows who Guy Penrod, Ivan Parker, or David Phelps really are or what made them significant in the Christian gospel industry.
But did I ever see any true image of Christianity in my grandparents home, beyond the lowest level, prayer at bedtime, maybe a short reading of the Psalms before going to sleep, no.
And yet I think they are probably saved. I might use the term "Theological Quotient". Simply put, not everyone has a high theological quotient. I don't think that bars them from living decent lives, loving the Lord, being somewhat attuned to Christian living, and language, and community. I just think it means they're not going to be passionate about studying the Bible, searching for answers, day and night, praying intensely for revelation. That's something that a person with a high theological quotient would do. But perhaps, maybe, ordinary Christians are not called to that. Still I think most Christians are totally disengaged, but, this theological quotient will be a helpful instrument for us in the future to examine the body.