How old will you be when you die? You may not worry too much about this (I hope you don't), but life insurance companies sure do. They stay in business by figuring out the survival chances of their policyholders beyond a certain age. They're experts at this because they have to be. If their insured population suddenly started living an extra year or two beyond expectations, the companies would be go bust.

Here's a very simplified version of an "actuarial" chart used in the insurance business to calculate risks and premiums. At birth, in the U.S., the average life expectancy for women and men combined is 78.1 years (with women having a four-year advantage over men, sigh). So if you live to 50, does that mean you can look forward to another 28 years? Heck no, if you've made it this far, you can expect to die at 85.4, seven years better than your at-birth average. And if you do make it to 85.4, give yourself another nine years - 94.4 is now your average checkout time. You're 100, you say? Congratulations, give yourself another three years. [CHART 1 NEXT TO THIS PARA]

The sensitivity of life expectancy to present age can lead to misleading statistics. For instance, a University of Texas research paper showed that the life expectancy at birth for Roman citizens was about 25. However, if civis Romanus made it past the first five high-risk years (infant mortality was huge back then), life expectancy was 48 (compared to 76 for a U.S. 5-year-old today).

Another way to look at mortality rates is to consider how many of us survive to a given age. So in the U.S., again combining women and men, 98 percent of us live to age 30, while 88 percent of us live to 60. Yet another approach is to use a "Gompertz" mortality curve (after mathematician Benjamin Gompertz, 1779-1865). This one, for the U.S., shows that your chance of dying during, for example, your 30th year is 1 percent; during your 60th year, your odds are about 10 percent. [CHART 2 NEXT TO THIS PARA]

The numbers are changing all the time, in our favor. We're living longer, on average, than our parents, who lived longer than their parents. While the demographic numbers are skewed artificially (as it were) by, for instance, two world wars and the boom in birth rates after 1945, we now live in a time and place where antibiotics, childhood inoculations, early cancer screening, heart bypasses and the like give us extra years over our ancestors.

Yet another way of looking at this is the "crossing the freeway" model. Life consists of running across a busy freeway (remember Gattaca?) where each lane/decade has more traffic than the previous one. That's with the exception of the first to second lane where infant mortality skews the picture, equivalent to the initial dip in the Gompertz curve. [DRAWING 3 CAN GO ANYWHERE]

And with no pedestrian islands between lanes - good luck!

Barry Evans (barryevans9yahoo.com) has never known a time when he wasn't alive. He plans to keep it that way.