Since becoming an adult, I've ridden a mountain bike in Oak Mountain State Park for a total estimated distance of about 3,000 miles (that's about 16 million feet). For most of that 16 million feet, my helmet did absoulutely nothing for me. But for that 20 feet (+/-) I travelled during my crash where I landed on my head/shoulder, the helmet saved me major injury and possibly saved my life.

I'm "lucky" that I never happened to have a 20 feet of travel like that during my rides as a teenager.

So from my point of view, riding a bike without a helment is like playing a game of Russian Rulette... except rather than one gun with 1 bullet among 6 chambers, your playing with a thousand guns, only one of which is loaded with 1 bullet. With that setup, the odds are that you will come out of that game alive. But the same can not be said of everyone that plays the game.