I would rather share the fate of my maternal forebears — old old age with an intact mind in a ravaged body — than the fate of my other grandmother.
Like Susan Jacoby, I actually hope I don't live to such an old age. And I share her conviction that diseases like Alzheimer's can allow one to live too long to live well. But as I've mentioned before, I developed rheumatoid arthritis when I was in my late twenties. And even though I'm fine now thanks to medication (which could, of course, destroy my liver or cause cancer, but I'll cross that bridge if or when I get there), I spent three and a half years in unrelenting physical pain as doctors puzzled over what was wrong with me if my blood tests kept coming back showing a negative rheumatoid factor. Thus, I can report with confidence that even with the overall good health of a twenty-something to alleviate at least some of the decrepitude, it's not easy to live that way either. The intact mind tends to be a bit too preoccupied with the ravaged body to spend much time enjoying the fruits of contemplation. So it's more like "six of one, half-dozen of another" if you ask me; a life as a physical or mental ruin with no reasonable hope of improvement is a life worth ending on its own terms.
I don't say it to be morbid. In fact, I think contemplating one's own mortality, while initially terrifying, of course, leads one to an appreciation of life that doesn't require the sorts of fantasies Jacoby mentions, of people determined to live longer than a century, a century and a half. Those people, to me, don't understand that life's finite limits are actually what give it meaning in the first place. As Steve Hagen said, we want it because it dies. And let's be real; if people thought they could solidly expect another fifty or so years of life, what would they do with that time? Travel, learn new languages, create art? Or spend that much more time eating junk food on the sofa while watching TV? The problem isn't that we don't have enough time, it's that we take the time we do have for granted. That's not going to change by adding on another few decades.