Editor's note: The above video originally aired in January 2019.

New guidance on concussions shows there isn't enough solid evidence to answer some of parents' most burning questions about contact sports. That includes what age is safest to start playing them.

Pediatric experts in sports medicine and brain science evaluated decades of research.

They say recent evidence filled in some blanks. But it's unclear how many concussions are too many and what are the long-term consequences of multiple concussions in youth sports.

The advice was published online Monday in the journal JAMA Pediatrics. Dr. Frederick P. Rivara, professor and vice chair of academic affairs in the University of Washington Department of Pediatrics, was the lead author.

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