According to this article, high school players are more likely to experience concussions-
http://www.nytimes.com/2013/10/31/sports/football/nfl-roundup.html?src=recg
I would add that in my experience it is possible that young players do not have the awareness to fully understand, or realize that they have sustained a concussion, or feel compelled to inform a coach or guardian of the event, choosing to brush it off and play through it.
To learn more about the ELDOA for concussions see Legacy’s past posts on this topic- www.legacyperformwell.com