The major reason is the loss of muscle mass. As we age, we lose our muscle mass, which slows down our metabolism and we begin to burn fewer calories. Both these factors make it harder to shed kilos, especially from the midsection.
Another reason is the hormonal changes. Both weight gain and weight loss are driven by our hormones. Women generally store more subcutaneous (soft) fat in the abdominal and thigh area and men store more visceral (hard) fat in the abdominal area. This starts happening when they hit puberty and remains the same till they reach 40. After this age, hormonal changes take place in both men and women. Women go through perimenopause and menopause and the fat storage tendencies shift. They gain more hard abdominal fat due to lower testosterone and estrogen levels and men gain more soft fat.