Livestrong.com

Understand Why Women Gain Belly Fat

As you age, you can't avoid gaining some belly fat and you won't be able to lose all of it, but you can minimize its appearance and its negative health effects with lifestyle and dietary interventions.

Starting at age 30, you begin to experience sarcopenia, which is the natural loss of muscle mass that naturally occurs with aging. The American Association of Retired Persons states that this loss is approximately a pound a year after 30 -- if you don't engage in strength training to preserve your muscle tissue. By 60 -- with no exercise -- you may have lost about 30 pounds of muscle mass, replacing the muscle mostly with fat. Fat is less efficient at burning calories than muscle is, which means that your body's resting metabolism also declines. So, even if you eat the same amount of food as you did when you were younger, you will likely gain weight. 

To prevent weight gain with your slowing metabolism, you need to reduce your daily caloric intake by about 100 calories every 10 years after age 40. So, at 60 or older, you should consume at least 200 fewer calories every day than when you were in your 30s.