In an article on the limits of the aging body, David Robson on the BBC.com Human body: The ‘ultra-athletes’ aged 60+ answers this question with a surprising result. It is not the cardio-vascular system nor the stress on the joints (although my body would beg to differ). Strangely, according to Robson, the brain may be the body part that suffers the most.
"Strangely, the brain perhaps suffers the most. One fMRI study, which scanned the brains of athletes (young and old) taking part in a 4,500-kilometre (2,800-mile) ultra-marathon found that the brain’s volume of grey matter fell by 6% across the course of the race. “It’s a very profound loss,” says Wolfgang Freund at Ulm University Hospital in Germany, who followed the athletes with a 50-tonne truck full of brain imaging equipment. However, it returns to its usual size during the following months. For this reason, Freund suspects the cells themselves weren’t dying, but shrinking as their nutrients were drained to feed the rest of the body. “The body is sucking everything it has to burn on the road.”"In regards to the aging question, the author says athletes over 60 still do well in these events, despite slower speeds and longer recoveries, although he doesn't acknowledge the fact that there are athletes who can no longer participate in these events due to the excessive mileage and deterioration on of joints and muscles that can no longer perform. Those over 60 still competing are the survivors or newer converts with less body mileage. In fact the author thinks that the older athlete may even do better in these events because of their stronger mental abilities.
The article is a great read whether you are over 60 or not and even if you have no desire to be an ultra endurance athlete. The takeaway is that you shouldn't be thinking about tapering your activities as you get older like we are sometimes told. Keep pushing those boundaries! I wish I had books like this years ago, before destroying my body doing marathons and triathlons: Build Your Running Body: A Total-Body Fitness Plan for All Distance Runners, from Milers to Ultramarathoners - Run Farther, Faster, and Injury-Free Maybe I would be still be looking forward to hitting and competing at age 60 in a few years.