Unfortunately there is no simple answer to this issue. No magic pill, no magic diet, and no magic requirement. To live a long healthful life, there are just a few simples rules to live by. And even then, you're not protected from accidents.
Diet
Undoubtedly, diet is an issue. However, it is usually not as big of an issue as some make it out to be. There is no perfect diet, or correct diet that will ensure a long life. Just like smoking isn't necessarily an ingredient for a shorter life-span. There are people that have lived well into their 90s having smoked every day since their teens. But since most people don't have this sort of immunity, it's best to keep the cleanest, most nutrient-dense diet you can. This means avoiding chemical-laden highly processed foods. Artificial sweeteners, flavor-enhancers, and other unnatural foods should be avoided as much as possible. Also, vegetables and fruits should be added in high amounts. A diet high in plant foods has been shown time and again to be beneficial against almost all chronic diseases (obesity, diabetes, heart disease).
Attitude
Happy people are healthy people. It doesn't take a genius to figure this one out. On countless anthropological studies of healthy populations, those individuals which lived longest also had the best attitudes toward life. They were happy, easy-going, loving, and positive. Even with the worst diet and living habits, a positive attitude can have a tremendous effect on lifespan. Learning to accept life the way it is and not take thing too serious are two big steps on the road to better emotional health.
Exercise
Another feature in the lifestyles of the healthy is constant exercise. Whether it be walking up and down stairs, working in the fields, or playing a sport, exercise is necessary to keep the body in good, youthful condition. Even with the best diet, attitude, etc, you cannot reach maximum age capacity without working out your body.
Restricted Calorie Diet
This is somewhat controversial, however it has been shown to be true in studies, and in individual cases. Individuals which restrict their calorie intake tend to live longer than individuals that don't. The truth is, that extra fat you carry around is very draining on the body. A person with the same body composition with more or less fat than you will undoubtedly be putting a different amount of pressure on their body to function. Fasting is also infamous for extending peoples' lifespans. In certain populations in the world, a yearly fast or period of slight starvation is hypothesized to promote the long lifespans of these people.
