It's important to distinguish between life span and life expectancy. Life span is characteristic of a species -- two or three years for mice, forty-five years for chimpanzees, and 110 to 120 for humans, although there have been reports of humans living even longer, but most scientists don't believe them. A fifty-year-old woman today has a life expectancy of eighty-five, and if you're younger than fifty you could hit ninety.
Life expectancy for a species is based on projections of how long 50 percent of a group will survive. Until about 1850, life expectancy for humans ranged between thirty and forty years. Then, in 1900, a person born in a developed country could expect to live an average of fifty years. By 1946, life expectancy had risen to sixty-seven, and today it averages out at about seventy-four.
While most people credit medical advances since the middle of the nineteenth century, evidence indicates that improvements in transportation (enabling more intermarriage and strengthening of the gene pool), nutrition, sanitation, education, and housing are largely responsible for the dramatic increase in life expectancy.
Slowing the clock
For centuries man (and woman) has searched for the elusive fountain of youth. Up to a point it can be achieved, and I'm going to show you some ways in which you can at least slow the clock. First of all, it's important to understand that aging and disease are two different things. Aging is the mysterious process that inevitably leads to death, while disease can be the result of individual lifestyle. It seems logical therefore that we can slow down the aging process by staying healthy and thus often prevent degenerative diseases.
Certainly, at thirty-five or forty you're not going to have the dewy cheeks of a twenty-year-old, but through discipline and knowledge you can keep that twenty-year-old skin and body looking as good as possible for years to come, and it's never too late to start. No one says it's easy, however. Scientists today increasingly report that debilitation in middle and old age is mainly caused by improper nourishment, poor eating habits, and lack of exercise. There are few shortcuts to aging well and if you want to get the best out of life after middle age, you should start working towards that goal from a relatively young age.
However, if you're not in the first flush of youth don't despair. Even sixty- and seventy-year-olds can see a huge improvement through watching their diet carefully and exercising regularly. You've got to use it or lose it. Unless it's taken care of, nothing will work properly or wear well if it's not used and this is particularly true of the human body. Try not speaking for a few days when you've had a sore throat. When you next attempt conversation, you will be hard-pressed to make your tongue and vocal chords do what they were used to doing and the longer you don't talk, the longer it will take to get back to normal.