Generations of high school children gasp when they read Shakespeare's Romeo and Juliet, for they are amazed to discover that Juliet was only thirteen years old. We sometimes forget that, for most of human existence, our lives were short, miserable, and brutish. Sadly, for most of human history, we repeated the same wretched cycle: as soon as we reached puberty, we were expected to toil or hunt with our elders, find a mate and produce children. We would then have a large number of them, with most of them dying at childbirth. As Leonard Hayflick says, "It is astonishing to realize that the human species survived hundreds of thousands of years, more than 99% of its time on this planet, with a life expectancy of only 18 years." Since the industrial revolution, thanks to increased sanitation, sewage systems, better food supplies, labor-saving machines, the germ theory, and modern medicine, our life expectancy has risen dramatically. At the turn of the century, the average life expectancy in the United States was 49. Now, it is around 76, a 55% increase in a century. As Joshua Lederberg notes, "In the U.S., greater life expectancyâ€¦can be attributed almost entirely to this mastery of infection, this annihilation of the bugs." And today, the fastest-growing segment of our population is the group that is over a hundred years old."