I think about this a fair amount, perhaps too soon since I'm in my 30s. I don't think any of us wants to end up as a near-vegetable for the last years of our life. So if that's the case, why doesn't everyone just "accidentally" fall off a nearby cliff during a nature walk, once they realize they are getting too old to live independently anymore? It seems that we're prevented from doing this by a couple things.
Firstly, I think we come to that point and then we think about family, and we realize that we can't bear to make our children and grandchildren sad. We feel like we owe it to them to keep on living. There's a sort of external will to live that feeds into us through the existence of family and friends.
Secondly, I suppose that when one gradually ages to the point of decrepitude, one's standards for living decline so gradually that it doesn't seem so bad to be in a bed having people care for you, because it's only a small step down from when you lived on your own but could hardly get out of bed, and you had to have every meal either delivered to you or eaten out of a can. Perhaps we'll even feel that we deserve to be doted on and given some end-of-life care, having lived the long life that we did.
As for wanting to tell our story, I think we can do that now, here, or in a book as Terry has. As long as we have sufficient motivation, there won't be anything preventing us from still telling our story up until the day that we can't talk or remember anything anymore. I just think that most old folks honestly don't have a strong desire to tell their story anymore. But look at the ones that do. Hayao Miyazaki, the most important animation director of our modern era, has retired about six times. He's tired and he wants to stop working, but he always comes back to say one more thing. Even now in "retirement" he is drawing a new graphic novel.
I have a feeling that even when it takes a year of lying in a bed to die, that time passes by quickly, especially since many are no longer very mentally active by that time. And age seems to speed up time. I can see the years going by much faster than as a child. I can't imagine how it will be when I'm 80.