If your preteen wants to play American football, talk to some pros who started that early. Those who got into the sport before age 12 struggle on memory tests, a new study shows.

Pee wee football hits can be so rough they have generated a genre on YouTube, including this collection of clips by “Marco M74”:

“Our study suggests that there may be a critical window of brain development during which repeated head impacts can lead to thinking and memory difficulties later in life,” said one of the researchers, Robert Stern, in a press release.

“If larger studies confirm this association, there may be a need to consider safety changes in youth sports,” said Stern, a Boston University neurologist.

In the leading U.S. children’s football association, Pop Warner, safety concerns prompted a rule change in 2012.

Counting Pee Wee Football Hits

Stern and his colleagues published the study today in the journal Neurology.

The researchers tested 42 former NFL players with an average age of 52. All of the participants had experienced memory and thinking problems for at least six months. Half of the players participated in tackle football before the age of 12 and half did not. The number of concussions sustained was similar between the two groups.

Compared with those who started older, former players who started before age 12 performed significantly worse on all test measures. That was true even after researchers took into account the total number of years of football played and the age of the players at the time of the tests.

For example, those who played before age 12 recalled fewer words from a list they had learned 15 minutes earlier. And they made more repetitive errors on a test of mental flexibility, compared with those who started playing at age 12 or later.

Both Groups Below Average

The differences between the two groups represented approximately a 20-percent difference in level of current functioning on several measures. Both groups scored below average on many of the tests.

Pee wee football hits could be taking a significant tole, writes Christopher M. Filley, a neurologist at the University of Colorado School of Medicine in Aurora, in an accompanying editorial.

Football has the highest injury rate among team sports. Given that 70 percent of all football players in the United States are under the age of 14, and every child ages nine to 12 can be exposed to 240 head impacts during a single football season, a better understanding of how these impacts may affect children’s brains is urgently needed.

Because the study focused on NFL players, the results may not apply to the general public, Stern said. He called for more research before any new policy changes. “There are tremendous benefits of participating in youth team sports. The goal is to make them safer.”

The researchers took into account the total number of years of football for each player, But they couldn’t count the total number of head impacts.

“So it’s possible that the number of impacts is responsible for the reported results rather than the early age of exposure to football,” said Filley.

The study was funded by the National Institutes of Health, JetBlue Airlines, the National Football League and the NFL Players Association.

The photo featured on this page is by Emery Way.