50 percent of all people who have ever lived to reach 65 years of age are living right now. According to studies, human lifespan at birth has increased since the Upper Paleolithic period, where the average lifespan at birth was only 33 years. Longevity then decreased to 13 years in the Neolithic period. An additional 15 years added to the average lifespan at birth during the Bronze and Iron ages. This seesaw in average lifespan at birth continued until the early 20th century, where average lifespan at birth reached 30-45 years of age. The current world average lifespan at birth for the year 2010 is now at 67.2 years.
Life expectancy is the expected number of remaining years of life at a given age. Life expectancy at birth can be subject to misinterpretation because it is highly sensitive to mortality rate in the first few years of life. A population can receive a low life expectancy at birth if it has high infant mortality rate at any given time, even if all the surviving people get to live to be 65 years old. Iinfant deaths count as zero life expectancy at birth, and is averaged with the remaining life expectancy at birth of the population.
Women tend to have higher survival than men at any given age. Starting from fetuses, males have a higher mortality rate than females. Life expectancy at birth of women are also higher than that of men for almost all countries. For example, the difference in life expectancy at birth between men and women in Japan 8.1 years.