Throughout history the average life span of humans has been gradually increasing. Neanderthals lived to the "ripe" age of 20, the Ancient Greeks averaged 20-30 years of life, people in pre-Columbian America lived 30 years, in medieval England 25-30 years and at the onset of the 20th century the average life span was 30-40 years. Nowadays, the worldwide average life span is about 66 years. This figure is very much influenced by extremities, as Sub-Saharan regions in Africa skew this number down with an average life span of 45-50 years. In contrast, in developed countries such as US the average is about 77 years, almost double the number of years people expected to live 100 years ago. So what led to this dramatic increase over such a relatively short time period?
Several factors contributed to this increase. First and foremost we must credit discoveries in the life sciences, which set the ground for the development of life-saving therapies. Most notable are antibiotics for bacterial infections, vaccines for a variety of fatal infectious diseases and other life-saving medicines and medical procedures for a multitude of diseases. The availability and wide usage of the benefits offered by modern medicine are so significant that today most people in the Western World die of age-related diseases. You may think that this is rather trivial, but in fact this is a monumental historic achievement. Despite the fact that people who lived a hundred years ago were not genetically different than us, their life expectancy was 30-40 years shorter! Hence, it is very unlikely that they died of old age at 35, just as we don't die of old age at 35. So what did they die from? Diseases, malnutrition and others. Modern medicine "artificially" keeps the human body alive for 77 years at the cost of exposing us to new age-related health problems. The medical fight against these problems is currently focused on treating the symptoms of old age rather than slowing down or preventing the aging process. Nowadays, however, most scientists are trying to find ways to delay the aging process instead of elongating lives that are sometimes of very poor quality.
Harry Patch, the last surviving First World War trench fighter, here at the age of 109 (two years before he died). Adopted from Wikipedia.
Extrapolation of life span graphs in the last 60 years has led demographers to the conclusion that within just a few decades more and more people will live to the age of 100-110, this in spite of the fact that today only 1 in 10,000 people reach age 100. Is this possible? A whole community of biologists thinks that it is. Life-elongating experiments performed in many organisms, from microbes to mice, suggest that this estimation is not unfounded at all. For example, studies in worms have shown that a decrease in the levels of an insulin receptor-like protein more than doubles their lives. In addition, providing particular strains of mice with a low calorie yet nutritious diet elongates their lives by 50% compared to mice living on a "normal" diet. Such treatments may similarly increase the human life span to 115-154 years! But of course, as is always the case with new and controversial fields of study, not all experts agree with these conclusions, arguing that the effects observed in some organisms are not necessarily relevant to humans. The most prevalent argument is that comparing short-lived species such as worms to long-lived organisms such as humans is simply wrong, since the aging processes are very different between the two. For example, worms can enter a hibernation-like state that is very significant for their longevity, a phenomenon that has no parallel in humans. In general, scientists have been most successful in elongating the lives of short-lived animals such as worms and flies (relatively to their normal life span). Why is that? It is possible that these organisms are more pliable to such manipulations, and that these manipulations can add only several months to an organism's life, which is of course more significant in animals that normally live only about one year. In humans, a several months' increase in life span is too little to consider significant due to person-to-person variability.