Life expectancy at birth has doubled in the past two centuries. This is largely due to advances in reducing childhood mortality: public health measures, control of infectious disease, and so forth. Adult life expectancy has increased more slowly, and remaining life expectancy in old age more slowly still - these are driven by new and more effective treatments for age-related disease, producing an incidental extension of adult life. The research community is only just now starting on the project of deliberately trying to slow or reverse the causes of degenerative aging, rather than focusing entirely on ways to fix the worst and most visible consequences of aging after they occur. This is why projecting past trends in life span into the future is not likely to produce accurate results - the entire approach to human medicine is presently shifting.
This article goes into some detail on the historical roots of modern gains in life expectancy at birth, much of which were a matter of better organization and sanitation rather than medical technology per se:
The most important difference between the world today and 150 years ago isn't airplane flight or nuclear weapons or the Internet. It's lifespan. We used to live 35 or 40 years on average in the United States, but now we live almost 80. We used to get one life. Now we get two. When I first started looking into why average lifespan has increased so much so rapidly, I assumed there would be a few simple answers, a stepwise series of advances that each added a few years: clean water, sewage treatment, vaccines, various medical procedures. But it turns out the question of who or what gets credit for the doubling of life expectancy in the past few centuries is surprisingly contentious. The data are sparse before 1900, and there are rivalries between biomedicine and public health, obstetricians and midwives, people who say life expectancy will rise indefinitely and those who say it's starting to plateau.
There's nothing like looking back at the history of death and dying in the United States to dispel any romantic notions you may have that people used to live in harmony with the land or be more in touch with their bodies. Life was miserable - full of contagious disease, spoiled food, malnutrition, exposure, and injuries. But disease was the worst. The vast majority of deaths before the mid-20th century were caused by microbes -bacteria, amoebas, protozoans, or viruses that ruled the Earth and to a lesser extent still do.
How did we go from the miseries of the past to our current expectation of long and healthy lives? "Most people credit medical advances," says David Jones, a medical historian at Harvard - "but most historians would not." One problem is the timing. Most of the effective medical treatments we recognize as saving our lives today have been available only since World War II: antibiotics, chemotherapy, drugs to treat high blood pressure. But the steepest increase in life expectancy occurred from the late 1800s to the mid-1900s.