"Some of the sports we've introduced children to involve hundreds of hits to the head," said Chris Nowinski, a concussion expert and author of "Head Games."

"Show me the scientific evidence that that's a good idea."

What everyone seems to agree about is that hits to the developing brain are bad; that agreement can dissolve once the emotionally charged conversation about the sanctity of sport begins.

"I would love to debate anyone, anytime, anywhere about this," said Cantu. "What the other side will say is you have to teach the kids the skills of a sport at a very young age otherwise they're at a competitive disadvantage. It's just plain B.S."

"I'm pro-sports. I just want them to be played more safely," said Cantu, adding that youth athletes have the least-experienced coaches and few medical personnel on the sidelines, putting them at a compelling disadvantage.

What he wants is for parents to carefully consider the potential long-term consequences of brain blows in their children -- to advocate for their safety.

"Reform should be happening much faster at the youth level than it does," said Nowinski, who is co-founder of the Sports Legacy Institute with Cantu. "We're saying wake up, your child has so many disadvantages.

"Your kids only have one life and you should think twice about putting your kid in a sport where they're hitting their head repeatedly."

As for the magic number 14? Cantu says that age matters less than maturity and that parents might even decide to wait until after age 14 to expose their children to contact.

"The mantra of the book is that no head trauma is good head trauma," said Cantu. The brain cannot be conditioned to take head trauma so make it as little as possible."

Questions lingers about long-term impact of subtle hits to the head