I'm 68 and I work out 6 hrs/wk (treadmill, rowing, swimming, lifting). Maybe I'm lucky but I never had problems with my back or knees, I did a lot of sports and farm labor/construction in my younger years (on weekends, I had a career as an engineer). I notice my bulk lifting power hasn't really decreased, honestly I could almost swim as far now as I did at 30 (2-3 miles). It is however easier to pull a muscle (and in my case get a hernia, I've had 2 in last 10 yrs), so I have to be careful. In construction I've seen guys in their early 70s really get after it, and of course Jack LaLanne did amazing things late in life. I believe maintaining muscle mass gets more difficult in 70s when "T" declines, but those continuing 5-7 hr/wk workouts seem quite healthy and capable (most in their 70s don't work out nearly that much).
