People worry more about staying sentient and functional at an old age. This is because people always want to feel young or as youthful as they can. To not have these functions means for many that they simply don't "feel alive." For them that is a worse curse than actually not having life.
People worry more about staying sentient and functional at an old age, rather than dying, because death is inevitable. Staying functional is something that people can control. There is no sense in worrying about something that cannot be controlled. People want to pack all they can into the few remaining years of their lives.
Old people seem to be more concerned about their abilities and functioning than they do about death. I think this is because the closer you come to death, the less afraid and more understanding you are about the fact that you will eventually die. It would be more important to me to spend my last days functioning and talking to those I love than worrying about when I would die.
The ability to understand what is happening around and react to it is more important than life to the elderly. What is the purpose of living if you can't enjoy even the littlest things. The ability to enjoy food brought to your mouth by your own hand, or laugh at a joke told by a familiar face is all that most elderly want. If these simple things can not be enjoyed, what is the point of living?
I am not old, but one thing I want when I am old is to stay sentient. I believe many old age people desire to live healthy until they die. I don't think many worry about death, since that is inevitable and is bound to happen to every human being. They worry more about staying sane and being able to see, hear, and feel things properly as they age.