because our metabolism rate goes down as we get older, most of us older guys need to crank that metabolism up with exercise in order to stay fit. that means getting your heart rate up to 60 - 80 percent of your maximum heart rate (based on age and weight). you can look up what your max rate should be on the internet.
if you get your heart rate up to 60 - 80 % for at least 30 mins a day, you are stoking up that furnace, and your elevated metabolism will continue to burn calories at a higher rate for the remainder of the day, even when you are resting.
diet alone won't cut it when you are older. but when you combine a reasonably healthy diet with regular exercise, you will see results and get the bounce back in your step.
it is kinda funny to see people circle a parking lot endlessly to get 10 feet closer to an entrance, when a little walk might do them some good...