Sounding another warning for parents of small children who play football, Boston researchers studying the donated brains of football players say they've found that playing tackle football before age 12 was associated with earlier onset of cognitive and emotional symptoms in those players.
"Youth exposure to repetitive head impacts in tackle football may reduce one's resiliency to brain diseases later in life, including, but not limited to [chronic traumatic encephalopathy]," Ann McKee, director of Boston University's CTE Center, and senior author of the study, said in a statement. "It makes common sense that children, whose brains are rapidly developing, should not be hitting their heads hundreds of times per season."
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The findings, from researchers at the VA Boston Healthcare System and the Boston University School of Medicine, were published in the journal Annals of Neurology.
The study looked at football players who died and whose brains were donated for research at BU. One focus was on a group of 211 who were diagnosed with CTE (plus other brain diseases, in some cases).
Researchers conducted online or telephone interviews with family members, friends, or both, of the dead players to find out when they had begun showing cognitive, mood, or behavior symptoms, the university said.
They said that every year earlier that the players with CTE began to play tackle football was associated with an earlier onset of reported cognitive problems by 2.4 years, and behavioral and mood problems by 2.5 years.
Looking at players who started before 12 and players who started after 12, researchers found that the former group had symptoms appear on average about 13 years earlier than the latter group.
"While participation in sports has important health and social benefits, it is important to consider contact and collision sports separately and balance those benefits against potential later life neurological risks," the study's lead author, Michael Alosco, an assistant professor of neurology at the medical school, said in a statement.
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The study did not find any connection between starting tackle football earlier and the severity of the brain diseases, however, the researchers noted.
A Pop Warner spokesman said the organization hadn't had time to review the study. The organization said in response to similar research last fall that youth football has "evolved significantly" and Pop Warner has made it "safer and better than ever before."
The researchers noted a number of limitations to the study. One is that its results were "specific to this sample of participants . . . and may not generalize to the broader tackle football population."
The study did not include a control group, and the researchers said it could suffer because families might have been more likely to donate a loved one's brain posthumously if they suspected something was wrong.
Martin Finucane can be reached at martin.finucane@globe.com