Medical researchers took a bunch of old people, (older even than you, Steve, but not taller ), all of them over 65 and put them into an exercise program. They were able to significantly add muscle mass and strength. More recently, somebody looked at whether you have to lift really heavy weights to gain strength, as few reps with really heavy weights is the traditional approach to building muscle mass. They found that you didn't have to do that. More reps with lower weights worked just fine, and should be easier on joints. The key point is that whatever recipe you follow, you have to keep going with your set until you can't do any more. As long as you tire out the muscles (temporarily) they'll respond by getting stronger.