It is not any one specific food, but a combination of all the health-promoting, high-nutrient plant foods – leafy greens, beans, fruits, etc. that slows the aging process and protects us from disease. The accumulation of oxidative damage over time is known to be a significant contributor to biological aging, and oxidative damage is limited by eating plenty of antioxidant-rich produce.
Also critically important is not to overeat on calories. There is substantial evidence that caloric restriction in animals dramatically prolongs lifespan, and also that excess weight in humans diminishes lifespan.
I discuss my list of recommended Super Foods in my book Super Immunity; they are called G-BOMBS: greens, beans, onions (and garlic), mushrooms, berries (and pomegranate) and seeds (and nuts). The health benefits of these foods are supported by plentiful scientific research; they help to prevent heart disease, cancer, diabetes, and other chronic diseases that age the body and cause premature deaths. When you follow a healthy eating style focused on these foods, you will be getting a huge quantity and variety of phytochemicals, and you will be eating for superior health.