Metabolism slows by 1 to 2 percent each decade after age 30. When you’re young, muscle burns up to 10 times more calories per pound than fat. As you age, muscle metabolism decreases. So even if you maintain the same level of exercise and calorie intake, you tend to accumulate fat. Regular exercise can help offset reduced muscle metabolism and help you stay lean. So will choosing nutrient-dense, lower-calorie foods.