By Dr. Mallika Marshall, WBZ-TVBy Dr. Mallika Marshall

BOSTON (CBS) – We know the dangers of repeated concussions and Boston researchers just found a connection between the age former NFL players were when they started playing and how they think later in life.

Researchers led by Julie Stamm at the Boston University School of Medicine studied 42 former NFL players and put them through a battery of tests on memory, problem solving, and verbal intelligence.

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Professor of Neurology Robert Stern, PhD, who was involved in the study, said there were some surprising differences based on when the retired athletes began playing tackle football in childhood. “What we found,” says Stern, “Was that the former NFL players who started playing tackle football before age 12 had significantly worse cognitive functioning now as adults compared to those who started playing at age 12 or older.”

Dr. Robert Stern (WBZ-TV)

Dr. Robert Stern (WBZ-TV)

Professor Stern cautions it was a small study but stresses that younger brains are vulnerable to recurrent injury. “There’s a lot of scientific literature,” he explains, “That indicates that there is this very important period of brain development for boys, in particular, between ages 10 and 12.”

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And it’s not just concussions that pose a risk. It’s the recurrent jostling of the brain inside the skull, even if it doesn’t cause symptoms, that is concerning.

“It makes common sense to me personally,” continues Stern, “That exposing our children to repetitive hits to the head during a time when their brains are really actively developing that that may not be so good.”

Chris Nowinski (WBZ-TV)

Chris Nowinski (WBZ-TV)

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And it’s the reason that Chris Nowinski, head of concussion research for the Sports Legacy Institute and former Harvard football player, has warned parents. “I do think parents need to think long and hard about when they introduce their son to tackle football,” Nowinski says, “Because the younger they start, the more years they’ll play, the more concussions they’ll have, and the worse off they’ll be.”

Dr. Mallika Marshall