Not knowing exactly what you mean by 'older people' I'm hesitant to make a comment. However, hesitation is just a pause so here goes.
When you're older the things you did when you were young that no one noticed or thought were funny now become 'cause for concern'. I've always talked to myself, especially in the grocery store. These days I notice the sideways glances of people who hear me and I know they're wondering if I'm crazy. I've always walked into things but now people think I can't see where I'm going or I've lost my balance. I walked into a telephone pole once when I was 24 and people laughed, if I did that now they'd think something was wrong and want to call 911. I forget things, usually why I've walked from one room to the other but then I'll remember something else I wanted to do in the room so who's to say that wasn't why I walked into the room in the first place.
Not everyone experiences mental decline. I've met people who were almost 100 years old who were still as sharp as the proverbial tack. However, I have a theory about mental decline in older adults. They just get tired of thinking. They don't seem to be able to learn new things because they're just not interested. For a young person this probably doesn't make sense but if you think about it, it will.
Let's use me as an example. I'm 70, almost. I was born in 1946. There was no television. So when I was a kid, television was something new that I had to learn about and there was that ever present antenna. Then came color television, you actually had to adjust the color, but there was still that antenna. Then came cable or antenna. Most people stayed with the antenna because cable cost money and who in their right mind would pay for television. Then came only cable. Meanwhile our telephones went to crap because AT&T split up. Then computers came into the home. Then cell phones. Now streaming that, with any luck, will kill cable TV. The problem is that every time something new happened, we had to relearn everything we had already learned. And it's no where near over. When the people who are now 'older' were young the way things worked was you got your degree, you got a job, you worked, and you retired. There was none of this relearning every year because the entire world changed.
So people just get tired of having to relearn all the time and to a young person that appears to be mental decline.