Check out this WSJ article about getting older as an athlete. The author talks honestly and openly about his passion for competition, and how he had to come to terms that trying to come in first at 50 could be risky to his health when his doctor found an aneurysm in his aortic root. So that led him to do more research:
"'The no-pain-no-gain mentality suggests that you can keep making gains if you just work harder,' says Mark Allen, a 51-year-old athletic coach once known as the world's fittest man for winning six Ironman Triathlon World Championships. As co-author of a new book called 'Fit Soul, Fit Body,' Mr. Allen argues against fighting age with more hours on the treadmill. 'If you can't let up on the competitive part of it, if you have to go as fast at 50 as you did at 20, you will grind yourself into the ground and become stressed out, bitter and unhealthy,' he says."
So while recognizing that older folks are and can be physically active (and competitive!), we have to take care ourselves first and foremost - and sometimes that means not being the very best.