During the last century, the average life expectancy of an U. S. citizen almost doubled. If you were born in 1900, you could expect to live 47 years on average. An American born today can reasonably count on living to almost 80. Moreover, an American typically lives twice as long as a citizen of Afghanistan or Sierra Leone.
The reason, of course, is advances in scientific medicine and sanitation. When Florence Nightingale brought home the lessons she learned in the military hospitals of the Crimea, her influence was quickly felt in Europe and America. The English writer Lytton Strachey said of Nightingale that she seemed "hardly to distinguish between the Deity and the Drains," that is, between religious faith and scrupulous elimination of agents of infection. Only when the drains -- scientific medicine -- became paramount did hospitals enter the modern era and life expectancy begin to significantly change.
Antibiotics, rather than incantations. Vaccines, rather than charms. Antiseptics, rather than resignation. In the developed countries of the world, most of us now die of old age, something previous generations took as rare good luck.
Which is probably why the genes that cause senescence have not been deleted or modified by natural selection. In former times, no one lived long enough for those genes to significantly affect reproductive survival. Death began to reap its harvest starting at birth, and the number of people living to a given age steadily declined until old age took the few remaining survivors. The human survival curve was the same as that for sparrows or salamanders. Today we have an increasingly "rectangular" survival curve. Survival is virtually assured until senescence, then -- boom -- we fall off the edge.