There are many risks that affect a human-being's life span. They include smoking, drug usage, disease, the immune system's health and more. The leading cause of deaths in the US include smoking and obesity (obesity then breaks down into heart attack stroke, etc. while smoking also goes down into that but also includes lung cancer and etc.).
Basically, your life expectancy depends on your environment and mostly your actions. Many things that are in your control include drugs and exercise and eating habits. Using illegal drugs can severelydamage your physical and mental/emotional health therefore decreasing your time in life. Fortunately, those things are entirely in your control. Dangerous drug usage is 100% preventable, just simply be careful on your drug dosage, stay away from alcoholic foods, smoking, and other dangerous drugs, follow your doctor's instructions and you will be fine.
Your exercise/eating habits are unfortunately much harder to control. In order to control them or change them, you must "fight" your brain. When attempting to control them, your brain will use its senses to make you feel a certain way, or urge you do do something, such as eat or to skip an exercise. In addition, it may even control your internal organs to make you do your habits. In order to fight these habits, you have to fight your urges; one of the easy ways is to repeatedly tell your self in your mind that you have to do a certain thing, like to not eat as much.
An even harder factor to control is disease. It is your environment that mostly affects this factor but you can take some precautions to stay away from microorganisms. They include staying warm in cold weather. In addition, obviously, don't play with dirty items such as puddles, trash, and contaminated things. Also, when using hand sanitizer, make sure that if the germ killer is alcohol, be sure that at least 60% is alcohol, otherwise it will not be effective enough. Lastly, wash your hands if you think you have come across something contaminated, and take a shower every day with shampoo and soap. What if you already have a disease though? If the disease does not seem very servere, simply get an OTC (over the counter) drug that seems right for your situation (look at the drug facts to find out). If the disease continues for more than a week or longer than the amount of time you are supposed to take the drug (look at drug facts to find out), then see a doctor and follow the instructions your doctor gives you.