Depends on how one rides: Back when I was an edgy teen I accidentally rode too fast over a pothole right at the top of a rocky asphalt slope. I felt a whiplash on my shoulder, lost grip, rolled over three times down the slope, and slid head-first into a thick metal railing. I wasn't wearing a helmet and it was pure luck that I bent the head in time before hitting the railing. A bit faster and I would have cracked my head open; I've still got burn scars from that day.

It would probably help teens a lot if affordable helmets didn't look so goofy, it's the main reason we didn't want to wear them.