My Uncle Joe is always fantasizing about ways to outsmart Father Time. “Suppose you could reverse your aging process at some fixed point in your life,” he says to me, a crazed gleam in his eye. “So you could pick any age to turn the clock backwards and start aging in reverse. What age would you pick to try to maximize your life span?”  

In other words, suppose you pick age 75. That means you’d turn 75 and then start aging backwards another 75 years. So you could live up to 150 years.

Your risk of mortality increases with age, so picking a higher age doesn’t guarantee a longer life span. If you pick age 50, you’d live at most only 100 years, but your chances of dying before you hit the turnaround point are less than if you pick 75. It’s a safer bet, for a slimmer payoff.

Uncle Joe posed the question because he knew the answer to this puzzle hinged on probability, the bedrock of inferential statistics.

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