Ankle injury.

My son is 15 years old and plays football at a soccerdome indoor football arena. Last friday we dropped him off as usual and he met his friends who are all the same age. The soccerdome staff paired his five a side team with an under eighteen team and my son sustained a severe right ankle injury. I suggest that if my sons team was paired with a team of his own age then his risk of injury would have diminished. Your thoughts would be appreciated.