While I beleive that you should make noconcessions to age the reality is that in order to maintain a high level of fitness you have to adapt to your current physical abilities. I have become a master of the workaround. First of all I was an adult onset athlete, I started running and really working out around thirty, I'm now sixty two. Became a fifty mile a week competitive runner and run eight marathons including Boston.
Time does take a toll, however, and after achillies reconstructive and shoulder surgery as well as a hip replacement after a fracture running was pretty much a thing of the past. That hasn't limited me as a cyclist. I regularly ride six days a week and have just passed 9000 miles for the year. I work more on flexibility these days doing yoga and more body weight exercises than free weights.
I'm happy as long as I can finish a workout feeling like I've pushed myself and not just soft pedaled for an hour. So the bottom line is that despite some limitations I don't feel my age. I like to say you're only young once but you can be immature forever.