Better treatments for health care mean more Americans are leading longer lives.  Any individual's prospects for longevity are a combination of genes and lifestyle. 

Life expectancy at birth in the United States in 1901 was 49 years. At the end of the century it was 77 years, a whopping increase of 57%.  There are close to 70,000 centenarians in the U.S. today and a study from the United Nations predicts that number to increase to 1 in 100 of the boomer generation.

How Do We Age?
According to the 2006 edition of the Handbook of the Biology of Aging, aging is defined as the process of a system's deterioration with time.  How and when that happens to a human is individual.  Plus, the aging process for different body systems can occur at very different rates in different people.  Changes occur little by little, but ultimately, the heart becomes less efficient, bones begin to lose density and the number of neurons in the brain decreases (though in some areas of the brain the number of connections between cells actually increase).  Other changes affect vision, hearing, reflexes, metabolism and more. 

Why Do We Age? 
The short answer to why we age is that no one really knows for sure.  While there is no consensus over what causes aging, there are multiple theories.  At its most simplistic, two common categories of aging theory are:  

• Programmed theories.  Aging is predetermined and follows a genetically regulated biological timetable. 
• Damage theories.  Aging is predominantly a result of interactions with the environment and results from a continuous process of damage accumulation. The damage is a by-product of normal cellular processes or a consequence of inefficient repair systems.

What Determines Longevity?
While scientists continue to explore and debate the theories of how we age, they agree that aging and a predisposition to get or not get certain diseases have a strong genetic component.  For example, genetic controls can protect your cells from becoming cancerous.  However, the best data we have today suggests that only about one-third of longevity is due to genes.  Additionally, the importance of genetic inheritance matters less as we age.  By the age of 80, behavioral choices account almost entirely for overall health and longevity.   So, the aging process depends on a combination of both genetic and environmental factors.  One way to think about it is that genetics defines the potential for a long life and environmental factors determine your likelihood of reaching that potential.

Environmental factors that influence longevity are predominantly due to lifestyle behaviors – eating too much or too many foods that are bad for you, smoking, inappropriate use of alcohol and drugs, how you deal with stress, and whether you have strong social connections to family or friends.  Obesity contributes to the rise of cancers, heart disease and diabetes, and as it becomes more pervasive in the U.S., some predict that life expectancy may actually decrease by 2 to 5 years.  Cigarette smoking remains the single most preventable cause of death in the United States.  Other factors include what you do for a living, the quality of healthcare you receive, diseases you may have had early in life and many more.