What a great series of replies! They go deeper than I'd anticipated, and I'm really enjoying them. A little more info on me and my perspective, for whatever that may be worth.
I'm 64 and have some wear-and-tear issues along with the fallout from some injuries. Tinnitus plagues me (thanks to an artillery accident in 1974); how I would love to hear silence once again, but it'll never happen. My right shoulder is a Grade III separation (only one ligament intact out of the original three) thanks to an idiot who ran me off the road and off my motorcycle in 2008. Oh well ... it still works. Left shoulder has had minor arthritis or bursitis for at least that long. Learning good paddling technique (rotated forward stroke and the paddler's box) keeps the shoulders safe.
Last year I was hit with atrial fibrillation while breaking camp on Knife Lake. Two days, 18 miles, and 8 portages later I checked into the Grand Marais hospital and they cleared it up. When I got home, the ensuing stress test and echocardiogram revealed zero deposits anywhere near my heart, and the cardio condition of a 40-year-old athlete. I guess I'll keep riding my bike. (While I was dealing with a-fib, my 2-years-older brother was preparing for a quadruple bypass that saved his life.)
I'm careful, and that alleviates most risks. The key to that is mindfulness, having the discipline to focus on where I am and what I'm doing. That hasn't come easy and it breaks down sometimes. I talk to myself to slow my thinking down so I don't do something impulsive (thanks to partially-treated ADD). The two times I twisted my many-times-sprained left ankle in the BW were when I was distracted from watching my step. And I've learned a most important lesson about tripping and life in general: complacency kills. Any time I find myself thinking, "it'll be OK" the red flags start waving. That little thought is a cue that I haven't thought the thing through completely and am setting myself up for trouble. I wish I could say I've learned that lesson through observation, but the truth is it's come through difficulties, some of which were life-threatening.
The things I'm afraid of are spraining my right ankle, breaking a bone, and getting appendicitis. I have control over the first two and that's the best I can do. As for age-related risks, I don't know quite what to point to, which is why I raised the question in the first place.
Thanks again, folks. This is a neat way to get to know each other. What a great campfire conversation this would be!