While your metabolic rate does get slower with age, the process is not completely inevitable. "You can stop the decline by up to 80 percent with strengh training and aerobic activity," says Miriam Nelson, Ph.D., director of the Center for Physical Fitness at Tufts University in Boston and the author of Strong Women Stay Young (Bantam, 2000) and Strong Women Stay Slim (Random House, 1999). It is lean tissue mass that dictates how fast or slow your metabolism is: The more of it you have, the more calories you burn.