My grandfather died about a month ago. He was 94. My memories of him are mostly from when I was younger, when he was still quite sharp and independent, if somewhat limited by mobility issues.

Well, when he died, it seemed like the aunts and uncles who live near him (I have over twenty from that set of grandparents, lol) were almost relieved. They were not visibly sad at the funeral. I was, because I remembered proud Opa. But thinking about it, over the last few years, he has had both feet amputated and become almost completely deaf, and spent a lot of time in the hospital... it was probably really hard to watch him lose his independence, have to turn over his farm entirely, etc.

That said... Celia is right that sometimes people do get better, and there's no certain age where you are doomed to that fate. Some individuals lose the ability to care for themselves at age 30 (for any number of tragic reasons), and others are still feisty at 104. Maybe old age will be good to you, and you will enjoy chasing kids off your lawn well into your nineties.