Many economists are puzzled by the slowing of the growth rate in U.S. productivity. After all, productivity rose rapidly in the 1990s (2.2 percent annually), and it rose even faster in the first seven years of the new millennium (2.6 percent) before the financial crisis and the Great Recession. And one of the reasons, it was frequently argued, was that the widespread emergence of personal computers, once linked by the World Wide Web to a trove of data and information, made much work, much easier. But from 2007 to 2015, productivity growth sunk to only half of that rate, or 1.3 percent annually. Certainly, we're more "wired" than ever to the internet — even if there are no longer any wires.

So what gives? While the decline in the years 2007 to 2009 clearly may be explained by the onset of the Great Recession, what about the last six years — from 2009 to 2015? Why such slow growth now?

A big part of it lies in the recent and widespread loss of older workers in the workforce.

First, a few facts about how the Great Recession affected older workers: In January 2005, before its onset, older workers (55-plus) were 25 percent of the long-term unemployed group, or those out of work for 26 or more weeks. In January 2010, a full half-year after the Great Recession had "officially" come to its end, the percentage of long-term unemployed workers over 55 had more than doubled, and it had increased to 53 percent of all of those were out of work for 26 or more weeks. While the percentage of older workers among the long-term unemployed improved somewhat to "only" 41 percent in 2015, it remains the highest of any age group.

The sad fact is that this is being driven by some corporate employment policies. A USA Today story published in late August revealed a new lawsuit charging Hewlett-Packard (HP) with age discrimination in terminating four employees ages 52 to 63 as part of 27,000 job cuts announced in 2012. The plaintiffs allege that in 2013, HP's own human resources department issued written guidelines requiring that 75 percent of all new hires be straight out of school or "early career" job candidates. So the odds for older workers — or even somewhat experienced workers — who have since applied for jobs at HP are 3 to 1 against.

This isn't just a problem for the discarded older workers; it's a problem for the whole economy. A recent paper by Nicole Maestas of Harvard University, and Kathleen Mullen and David Powell of the Rand Corporation, is one of the first to tease out the impact of the loss of older workers on the broader economy. Their conclusion: On average, every 10 percent increase in the share of a given state's population over age 60 reduced per-capita productivity increases by 5.5 percent.

This came through two effects. First, as more workers retire, the labor force grows more slowly, thus dampening GDP growth broadly. But this explains only a third of the 5.5 percent reduction in growth. The bigger effect was through reduced productivity — that is, output per hour — of the remaining workers. The authors found that this couldn't be explained by emigration, mortality or an influx of younger, inexperienced workers.

So what explains the impact? As the authors note, "An older worker's experience increases not only his own productivity but also the productivity of those who work with him." All else equal, experienced workers are more productive, and their accumulated experience, knowledge and skill help all those who work around them increase their productivity as well.

The paper estimates that this continuing loss of older worker and their experience will cost the U.S. economy by reducing per-capita GDP growth by an estimated 1.1 percent in this decade, and its impact will continue to be felt into the 2020s, when growth will continue to be slower as a result of the continuing loss of experienced, older workers.

David Peterson, of Grand Marais, Minn., is an author and economist.