Proper heading requires a strong neck
As Webbe notes in a 2010 book,  "Generally speaking, there is general consensus that proper heading technique requires strong neck muscles to form a stable platform bridging the body and head."
A 2013 study  found that balanced neck strength may reduce the acceleration of the head during the act of heading a soccer ball, thus reducing the risk of brain injury from such low-grade, subconcussive hits. "Achieving and maintaining a balance in neck strength may be a key preventative technique in limiting acceleration, hence limiting the potential risks of repetitive heading in soccer," wrote lead author Zachary D.W. Dezman, M.D., a resident in the University of Maryland School of Medicine Department of Emergency Medicine.
In a 2010 clinical report on injuries in youth soccer (reaffirmed in May 2014), the American Academy of Pediatrics' Council on Sports Medicine and Fitness  recommends, rather than age limits, that heading of the ball only be taught when the child is old enough to learn proper technique and has developed coordinated use of his or her head, neck, and trunk to properly contract the neck muscles and contact the ball with the forehead. The AAP's position is similar to that recommended by Dezman and his colleagues in the 2013 study on neck strength in soccer.  "Balancing muscles may be particularly beneficial," writes Dezman, "for younger players learning the game and would perhaps be a more objective, quantitative parameter when deciding when to introduce heading," rather than strict age limits.
Since publication of the AAP report and the Dezman study, two more studies (both appropriately cited by SLI in its White Paper)[34, 35] have joined the growing body of evidence supporting the view that having a strong neck may help reduce risk of concussion. The first, conducted in a laboratory environment, found that adolescent soccer players with weaker necks experienced greater head acceleration during heading than those with stronger necks. The second, a field study involving over 6,700 high school athletes in boys' and girls' soccer, lacrosse, and basketball, reported that neck strength was a significant predictor of concussion, with the odds of concussion falling by 5% for every one pound increase in aggregate neck strength, and that the quarter of the subject group with the weakest necks suffered the greatest number of concussions, while the quartile with the strongest necks suffered the fewest.
Delaying heading: a mistake?
Beyond agreeing that neck strength is correlated with concussion risk, there appears to be little consensus on when heading should be introduced.
Arrayed against the position staked out by SLI/ISLE, Drs. Cantu and Webbe are some other "heavy hitters," including Dr. William Meehan, Director of the Sports Concussion Clinic and Micheli Center for Sports Injury Prevention at Boston Children's Hospital, and author of the 2011 book, Kids, Sports and Concussions,  who don't think there should be any age limits on heading and believe delaying the teaching of heading "would be a mistake."
Dr. Meehan argues that, as children become stronger and better coordinated, they are able to kick the ball at a much greater velocity. "It seems unwise," he writes, "to have their first time trying to head a ball occur at an age when the ball can be kicked with significant speed and force."
"Instead," Dr. Meehan suggests, "using smaller, softer balls that weigh less while children are younger allows them to develop the skills necessary for proper heading of the ball. This seems like a safer approach. They can learn proper technique, develop strength, and master the timing and coordination necessary for proper heading of the ball when young, before they begin to play with an adult-size ball that can be kicked with significant force." Learning to head with a dry, soft, foam ball may be another useful way to start, he says.
In this recommendation, Dr. Meehan finds support from an important ally: the American Academy of Pediatrics. Its 2010 clinical report,  the AAP recognizes that, while, proper technique as "foremost in reducing the risk of concussion from heading the ball," it also "is imperative that soccer balls be water-resistant [one of the supposed culprits in the brain damage found in early studies of retired Norwegian players,[8-12] see discussion below], sized appropriately for age, and not hyperinflated."