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This study has limitations. The brains used in this study may not represent those of the broader tackle football population, because most ex-football players, and their families, chose to donate their brains to shed light on their cognitive and behavioral struggles. Still, the results are alarming. “The data supports that you should not play tackle football until you’re more physically mature,” says McKee, whose future work will attempt to define a sort of tipping point for tackle football: how long can kids play before the risks rise exponentially?

Last week, California lawmakers dropped legislation banning tackle football for children under 12, and a similar measure has stalled in Illinois. But the momentum for limiting hits is expected to keep growing. “The cost-benefit of playing eight years before high school is so far on the side of being foolish, it’s inevitable that this change takes place,” says former San Francisco 49ers linebacker Chris Borland, who retired in 2015 at age 24 because he was concerned about the long-term effects of repetitive head trauma. Borland started playing football in the ninth grade. “The research is going to continue to come out. It gets harder to justify young kids playing.”

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"It shows the health of the brain was changed by playing football before the age of 12," McKee said.

"Some argue that players should play even later than 12, maybe 18, when they are adults and can make fully informed decisions," McKee said. "... The study adds to the accumulating evidence that if you're going to play football, you should do so later."

Doctors studied the brains of 246 former football players, 211 of which showed signs of CTE, a degenerative brain disorder that can cause memory loss and mood swings, among other symptoms. The study found players exhibited cognitive problems 2.4 years earlier per year they played tackle football before the age of 12. Behavioral and mood issues began 2.5 years earlier.

Some Pop Warner football leagues begin at the age of five. Based on the findings from this study, a player who began organized tackle football at age five could begin showing signs of cognitive degeneration 17-18 years before a player who began after the age of 12.

The study could continue building the groundswell of support for banning tackle football for children. Four states (Illinois, California, Maryland and New York) have considered youth tackle football bans this year.

None of the states have passed a law yet.