As a young man I didn't wear a helmet on bicycle or road motorcycle, on trail bikes I wore a helmet. A well dinged one I might add. Anyway on the bicycle I had at least two crashes that gave concussions. I don't know if there was any lasting effect, my wife might say there was. I know at my age now there would lasting effects. Anyway, there are so many variables in a crash that if things had gone just slightly differant there might not be any question about whether there were lasting consequences. How you get that across to a teenager is beyond me. Good Luck