A couple of days ago, I observed a milestone that I had been dreading for a while: I turned 59 ½.

The fact that I was even thinking of 59½ as a milestone isn’t as strange as it might seem. To begin with, when our kids were growing up, we initiated the custom of celebrating half-birthdays (with a half cake, naturally). Now that they and we are older, we’ve fallen out of the habit of having cake, or anything else special, but we still half-heartedly congratulate the honoree on reaching a semi-important date.

My reason for being concerned about this particular half-birthday was a little strange. Some years ago, I had read a book called “Predicting Presidential Elections and Other Things,” by Yale Economics Professor Ray C Fair. The book is a mostly non-technical introduction to the tools of econometrics, which, when used with large amounts of relevant data, can be used to evaluate how certain variables affect the likelihood of certain outcomes, for example, how economic data affects presidential elections.

After introducing the tools and methods for econometric analysis, the author then applies those tools to a number of topics, showing how one can make informed predictions about college football games, college grades (based on class attendance), wine quality (based on weather patterns), and so on. As an avid runner, the author also included a chapter on predicting how marathon race times decline with age. The chapter was meant to encourage aging runners, giving them a way of competing with their younger selves based on predictable rates of decline. We know this concept as “age-grading,” and it does provide some comfort when one can say that a race performance is equal or superior to one achieved in one’s youth based on being better than predicted after age is considered.

But when I had read the first edition of the book ten years ago, I had become focused on what struck me as an ominous observation in Fair’s analysis. After age 35 or so, the predicted rate of decline for race times is slow and steady. In other words, runners are expected to slow down at a constant rate of slightly less than 1% per year. But at a certain point, the rate of decline begins to accelerate. The author calls this the transition age, and it is at this point that things begin to get worse faster.

I had recalled from the first edition of the book that the transition age was 59½. It was at that point that a runner’s slow down would pick up speed, so to speak. The image of a cliff also came to mind.

Now at this point you might be growing impatient with this whole analysis. While the tools are useful for predicting average rates of decline and the downward curve of age group records, they cannot with any certainty predict how an individual’s performance will change over the years. An individual might be far more successful resisting aging than his or her peers, for a variety of reasons. Why be obsessed with averages, and with this virtual “cliff” when one could bend one’s own personal curve with smarter training and better decisions? All valid points, and yet they failed to dispel the dark clouds that seemed to gather as my half birthday grew closer.

Already preparing to be in mourning, I decided that I ought to re-read the book that had warned me about this phenomenon in the first place, so I went to my local library, and checked out the second edition “Predicting Presidential Elections.”

The first edition had been published in 2002, and the second edition had come out in 2012. In the 10 years between editions, the author had taken advantage of much more data, including several new world age group bests from Ed Whitlock, and had re-run the analysis of marathon times. I was stunned to read that the “transition age” – the age at which we begin to get worse much faster – was set at 75, not 59½. I had 15½ years to go before I headed over the cliff!

Had I, for all these years, misremembered the original transition age? It didn’t matter and I didn’t care. I had been given a reprieve, and could look forward to slow and steady declines of 0.8% per year for another decade and a half.

And fifteen years from now, as I approach another imaginary cliff, maybe the author will have published a third edition!