We all know that we should encourage our kids and grandkids to exercise because it boosts both their physical and mental health, but new research reveals that there’s a certain age at which the amount of physical activity that kids do tends to drop off the most.

And at that particular time in their lives, a few extra reminders wouldn’t hurt.

To learn more about exactly when—and why—this drop-off appears, I called study author Matthew Kwan, PhD, a postdoctoral fellow at McMaster University in Ontario, Canada, whose report was published in January 2012 in American Journal of Preventive Medicine.

THE SEDENTARY SLIDE

In one study, starting at ages 12 to 15, 683 Canadian teens were followed over 12 years and interviewed every two years. Each time, Dr. Kwan and his colleagues assessed how much physical activity the teens had done over the previous three months. Physical activity was defined as any mild, moderate or vigorous leisure activities—from bowling to soccer to marathon running—but did not account for activities such as walking to school.

The results: Over the course of the 12-year period, the amount of physical activity among girls declined, on average, by 17%, while physical activity among boys declined by 30% on average. Dr. Kwan said that it’s important to note that boys were more active to begin with at around ages 12 to 15, so that might explain why their decline was steeper.

Another study of Dr. Kwan’s looked at when the steepest declines within that time period tend to occur. He looked at boys and girls, both those who went to college and those who didn’t. He found that in three of the four groups, the steepest decline took place around the age of 18, as the girls and boys were finishing high school. Among girls who went to college, there was a decline in their exercise around that age, but it wasn’t quite as steep as that of the other groups—probably because they had had another steep decline in middle school.

WHY KIDS QUIT MOVING

So what’s so special about the age of 18? I have to admit that I was surprised by the results, since many college kids have so much free time! I would have guessed that the largest drop would have been seen right after college, when young adults enter the workforce, often in sedentary jobs, and start to take on many more of life’s responsibilities. But the results seemed more logical when Dr. Kwan shared the following insights…

The transition from high school to college usually means a big drop-off in the number of organized sports that students participate in. Even great high school athletes can’t always compete at the college level—either because they aren’t quite good enough or can’t take on the greater time commitment. Less glamorous “club sports” in college may not seem that appealing… so it’s all too easy for physical laziness to set in and for kids to say “I’m done with the sport.”

There are more distractions at college…and there is much more freedom to participate in unwholesome activities. For example, besides tracking physical activity, the researchers also asked the kids about how often they smoked and drank alcohol—and it’s no shock that those behaviors tended to peak during the college years. And if smoking and drinking aren’t anti-exercises, I don’t know what is!

HOW TO KEEP A SPROUT FROM BECOMING A POTATO

Is it all hopeless, or can we help our kids and grandkids keep up their health by keeping up their exercise? Well, Dr. Kwan said, if you have a bit of money to spend on this, indulge your older kid’s interest in, say, martial arts or dance by offering to spring for classes after he or she graduates high school. In many cases, that could be the best replacement for secondary-school sports. But spending isn’t the only answer, and not even the best answer. “There’s a lot of evidence that the more active parents are, the more likely their kids will be active,” Dr. Kwan told me. So, yes, if you really care about this, you should be a positive role model. Stay active yourself…make sure your kid knows that you are…and when your college kid is home on break, grab those hiking boots or tennis racquets and head out there together.

Source: Matthew Kwan, PhD, postdoctoral fellow, department of family medicine, Michael G. DeGroote School of Medicine, McMaster University, Ontario, Canada.