These metabolic differences alone, however, aren't great enough to account for weights that exceed the healthy ranges in weight tables. One explanation is that men have more muscle and less fat than women. Muscle uses more energy than fat. Because men have more muscle, they burn between 10 and 20 percent more calories than women during rest. As you get older, the amount of muscle tends to drop, and fat accounts for a greater percentage of your weight.
Metabolism also slows naturally with age.
Together, these changes reduce your calorie needs, often adding an extra pound a year after age 35. During the first few years of life, you form new fat cells rapidly. If a child gains excess weight in the first few years of life, they will form an excess of fat cells. In obese children, the number of fat cells is often three times that in normal-weight children. As a result, overfeeding children, especially in infancy, can lead to a lifetime of obesity.