Beginning at age 30, most of us lose about 1 percent - or a third of a pound - of muscle every year, as the body starts tearing down old muscle at a faster rate than it builds new tissue. (It’s why world weight-lifting records for the 60-year-old age bracket are 30 percent lower in men and 50 percent lower in women compared with records in the 30-year-old bracket.) The loss of muscle, which burns more calories than fat, slows the body’s resting metabolic rate, causing us to pack on fat pounds through the years. While we can’t completely halt this aging process, researchers believe we can do a lot to slow it down, mostly through resistance training, or weight training, that targets specific muscle groups.
-from "Stopping Age-Related Muscle Loss," Boston Globe, March 5, 2012.