When will getting old stop being interesting, and start being upsetting?

I'm going to die. I get that. And it's either going to happen after a long decline, or due to a catastrophe. Or decline followed by, but not caused by a catastrophe. Or catastrophe directly caused by a decline. But it's going to happen.Sucks. But interesting. At least to the present version of me.On the way to death things usually go downhill. I know people in their nineties who are still incredibly sharp. Maybe they aren't as good as they were in their fifties, but I didn't know them in their fifties. I just know them now, and I know that if I enter my nineties as sharp as they are, I'll count it as a win.But will I get there? And will I be (relatively) unimpaired. There's not a lot to go on. Here's some data, and a speculation:Datum: My dad died at 86 after a catastrophe. Years before he he'd had a stroke that crippled one side of his body and slurred his speech (probably due to a partly crippled tongue). His mind was not notably impaired, except for depression, reported by my Mom.Datum: My mom died at 94, after a long decline. It took away her short term memory but left all the positive parts of her personality intact. What she'd said in a conversation five minutes before was lost to her, by the time she died. The distant past was still clear enough for her to retell a semi-relevant story of something that happened thirty years earlier--sometimes three times in a single conversation.Speculation:Doing the math, I'll die at age 90 (the average of 86 and 94) with a quarter of my mind gone. (the average of Mom and Dad). The chances of my dying in a car crash before then have been reduced since the recent night that a cop pulled me over after I passed him doing 75. He'd probably been doing 60 on a road marked for 50. "Don't you think I was going fast enough?" He asked me. "I was stupid," I confessed. Several times. He told me to use better sense, and let me off. And since then I've slowed down.In the meantime I keep close watch for signs of cognitive decline. Bobbi and I alternate reading selections from "A Coastal Companion" at breakfast. Right now I do the odd days, she the even. When a month has an odd number of days, we switch. As I read I pay attention to my stutters, hesitations, and mispronunciations. They seem to be growing in number but I am an imperfect observer. There no way to determine to what degree my self-observations are causing my observed lapses. It seems to me that things have gotten worse, but there's no way to determine whether that's true.But if I have gotten worse, it's interesting rather than alarming. At least, right now. But will it stay that way? I imagine a future me who is unable to say three words without getting one wrong. Or one who has to speak slower, as I sometimes find myself doing, and who can't pick up the pace when he finds himself doing that, as I usually do when I notice.Present me hopes that future me will continue to find the entire process interesting. Present me hopes that future me will not find it depressing. But ya never know, do you?And that makes it interesting. At least to present me.