Your weight is determined mostly by three major factors:• The number of calories you consume everyday • The number of calories you burn through daily exercise • Your ageIf you eat too much and don't exercise enough, you will likely carry excess weight, including abdominal fat.Also, as you age, your muscle mass may decrease significantly as fat grows. Loss of muscle mass also affects your body's calorie-burning rate, making it more difficult to maintain a healthy weight.Even if they aren't gaining weight, many women experience a rise in abdominal fat as they age. This is likely due to a decrease in estrogen, which appears to affect the distribution of fat inside the body.There may also be a hereditary component to the propensity to accumulate or carry weight around the waist and to have a "apple" rather than a "pear" shape.