About a century ago, the average life-span for Americans was about 50 years. Today, the typical American lives for around seventy-eight years.

According to a German aging study, the maximum life span in industrialized countries has increased by two years every decade since the mid 19th century. What accounts for such increased longevity?

Between 1900 and 1950, inventions such as refrigeration and sewage treatment meant that young people were able to survive longer. Moreover, medical breakthroughs helped contain diseases such as polio, which killed many children. These advances helped increase the average life span.

Medical discoveries after World War II tended to benefit older people. Treatments for heart disease, for example, have allowed the elderly to live longer on average. Does this mean that future medical breakthroughs will result in even longer average life spans, or have we reached our limit? Scientists disagree.

Some argue that if science is one day able to eradicate disease and old-age infirmity, there will be virtually no limit on how long humans can live. Some even predict that by the year 2150, the average life span will have increased to around 120 years.

Other life-expectancy researchers find that scenario highly unlikely. Our bodies' cells can keep reproducing for only so long before they peter out. Only when science finds a way to keep our cells dividing longer will we see another significant leap in life expectancy.

Still, with plenty of exercise and a healthy diet, those so inclined can always hope that they'll live long enough to break the record held by Jeanne Louise Calment of France, who lived to be 122.