Article 1:

Is youth flag football safer than tackle?  The results of this study may surprise you.

http://journals.sagepub.com/doi/full/10.1177/2325967116686784#pq=X2hXJC

Conclusion:  Injury is more likely to occur in youth flag football than in youth tackle football.

Article 2:

From the Mayo Clinic:

http://www.coachwyatt.com/mayoclinic.htm

The risk of injury in youth football does not appear greater than the risk associated with other recreational or competitive youth sports.

Article 3:

An article quoting the Mayo Clinic

https://youthtoday.org/2013/02/the-myth-of-dangerous-football/

"Our analysis showed that youth football injuries are uncommon.

Article 4:

Injuries in Youth Football:  A Prospective Observational Cohort Analysis Among Players Aged 9 to 13 Years

http://www.mayoclinicproceedings.org/article/S0025-6196(11)61783-7/references

Article 5: 

NY Times: NFL Players Live Longer Than Other Men

http://www.nytimes.com/2012/05/09/sports/football/nfl-players-live-longer-than-men-in-general-population-study-says.html