The process for constructing survival curves is somewhat complex and requires specific data on the number of deaths of individuals in that population. The first step, is to lay out the format for a typical graph (i.e. an x and y axis). On the “x” or horizontal axis, plot the age in years of the population (0-160). Along the “y” or vertical axis, plot the percentage of survival.

In a typical survival curve graph, you would see the survival curves for white males taken at different time periods in different areas of the world. Note that in the third world, because of a large number of newborn and infancy deaths mainly due to infections and malnutrition, there is initially a steep drop in the survival curve. For the gerontologist, there are 2 interesting points on the curve: the mean or average i.e. the 50% survival point, or if you are a pessimistic you could also refer to it as the 50% death point. It also represents the point where 50% of the population has died. From that point if you drop a vertical line to meet the “x “axis (age in years) that point where it meets the x axis is designated the mean or average life expectancy in years for that selected group or population

For example, the above figure  taken from Walford pocket book  Maximum Life Span, p3, shows  the  survival curves of three different human populations. Using the above calculation,the mean live expectancy for the ancient Romans its 22 years , for a white USA male born in 1900 its  49 years and for a similar USA population born in 1960  its 72. For the year 1984 Walford gives a figure of 74. If we were to construct a similar graph for 2010 it would be around 78 years.   Currently in Canada  the mean life expectancy is marginally better then in the USA approaching 80. A white Canadian female currently outlives the  male by approximately 4 to 6 years. If this trend  of 2 years extension   for every decade of living continues then by the year 2050 the mean  life expectancy for the white North American male will reach 90 years.   Because of changing life styles for females like  smoking habits, drinking, frequency of auto accidents, obesity etc. their mean life expectancy in 2050 will only be marginal longer than for males.

The fourth curve in the figure labelled “Theoretical limits of biological longevity “ indicates the survival curve if we were to eliminate all premature causes of death like cancer, heart disease, stroke, diabetes, etc. Regardless of the group studied, including the last curve  all survival curves reach the horizontal or x axis at the same point, i.e. we all have the same maximum life span! 

Let me reiterate this a different way. Even if gerontologists could eliminate the major causes of premature deaths: the degenerative diseases like heart disease, cancer, diabetes, or the preventable diseases like accidents, they would only extend the average or mean life expectancy but not budge the maximum life span of 120. Indeed, eliminating cardiovascular diseases only extends the life span by roughly 10 years, while cancer eradication only extends it another 3-5 years. Even eliminating these diseases, especially cancer early in life (i.e. in childhood) you would only gain a few more years. Indeed, if we were to eliminate all the causes of premature deaths, we would extend the life expectancy to around 110-115 years, with a few stragglers reaching that maximum of 115-120, but almost never beyond 120.

Exceptions to the maximum life span are very rare. Supercentenarians are people who have reached the age of 110. There are about 90 verified living supercentenarians in the world. Jeanne Louise Calment of Arles, France had the longest confirmed human life span in history at 122 years, 164 days, exceeding the theoretical limit of 120 years.

Various gerontologists have already calculated the extension in mean life expectancy achieved by specific parameters like quitting smoking, losing weight, exercising, monitoring blood pressure and cholesterol levels, and other lifestyle changes. Currently, I am in the process of using these demographics and the internet to construct a functional biological clock. Current demographics estimate that the maximum life span increases by 2 years for each decade approaching 90 years by the middle of the century.