"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.