Life expectancy refers to the projected amount of years the population will live. In the 16th to 18th centuries, individuals were expected to live until the age of 40. Even as early as the beginning of the 20th century, an age of 45 was thought to be a long life, considering individuals from classical Rome and Greece only lived until about age 28. Thanks to modern medicine and better disease prevention in most developed areas of the world, the current world average is 67.2 years. In America, the average is 77.5.