“The brain is going through this incredible time of growth between the years of 10 and 12, and if you subject that developing brain to repetitive head impacts, it may cause problems later in life,” Robert Stern, one of the authors of the study, said of the findings.
111 N.F.L. Brains. All But One Had C.T.E.
A neuropathologist has examined the brains of 111 N.F.L. players — and 110 were found to have C.T.E., the degenerative disease linked to repeated blows to the head.
The study is consistent with earlier findings by Stern and others that looked specifically at N.F.L. retirees. That research found that retirees who started playing before 12 years old had diminished mental flexibility compared to those who began playing tackle football at 12 or older.
A growing 
number of scientists argue that because the human brain develops rapidly at young ages, especially between 10 and 12, children should not play tackle football until their teenage years.
Last year, doctors at Wake Forest School of Medicine used advanced magnetic resonance imaging technology to find that boys between the ages of 8 and 13 who played just one season of tackle football had diminished brain function in parts of their brains.
The N.F.L., which long denied that there was any link between the game and brain damage, has in recent years been promoting what it considers safer tackling techniques aimed at reducing head-to-head collisions.
More recently, the league has been promoting flag football as an even safer alternative, an implicit acknowledgment that parents are worried about the dangers of the sport and turning away from it.