I found an important study on Clarence Bass’s site here. A study of strength training in retirees revealed that intensive training is essential to preserve muscle mass. The study compared a group performing intensive resistance training to a moderate-intensity group, as well as a control group that did not exercise. Muscle mass was only effectively preserved for the high resistance group after four years. The high resistance group used weight machines, while the moderate group used resistance bands and body weight exercises. I would like to clarify that you can still lift heavy using resistance bands, you just need to use more of them.
Ultimately, go heavy! Clarence points this out all the time and I’ve mentioned it once or so. It’s nice to see this proven, especially for us older people, in a clear study like this.