As you age, metabolism slows down ... given the intake of food and activities in your life remain constant, you'll burn less, so turned into fat. Once you gain weight, it's hard to lose weight because the cells built to store the fat remains in your body. So the moment you have extra calories, those become fat.
It's likely you have a slow metabolism but how much exercise do you do? Try food that speeds up your metabolism such as spice - chillies. Also, drink loads of green tea - it helps you burn calories faster when exercising.