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My girls, aged 15 and 12, both wear skateboarding helmets for biking. Our youngest has the one with the skull and cross bones. We have the rule: we pay your health insurance, you live in our house, you wear a helmet. We let them pick their helmet, regardless of the price. I understand the need for fashion as well as function, and so our compromise was that we decided to forgo to free helmets from the fire department, and buy the kids the cool helmet of their choice. So far not wearing a helmet hasn't been an issue.
Honestly, though, if it does become an issue, I'm not sure how I will handle it. I will probably mention that, if you can't use good safety judgement and don't handle peer pressure well, that doesn't bode well for getting other privileges, like driving. If a couple of kids can make you throw caution to the wind with your BIKE, do you think I'm ever letting you drive a CAR? You really don't want to wear your helmet? Fine, I can't make you. I'll buy you whatever helmet you want, I'll help you explore and try to deal with whatever might be preventing you from wearing a helmet. But if you continue to make poor decisions like not wearing a helmet because peers say it's stupid, you'll never drive my car, and I sure the hell won't be helping you buy one or pay for insurance.
I agree with Linda, it's his body until his parents are stuck caring for him for the rest of his life because of a traumatic brain injury. Sorry, I'm a nurse and I take care of these people from time to time. Not just the ones who are paraplegics, with bedsores and having to be cathed every 3 hours for urination; also the ones who can't get a job, who have irratic behavior and memory problems, who have subsequently developed drug and alcohol problems, due to a head injury that "didn't seem that bad" at the time. You bet I tell my kids about people like this.
Sorry, your right to do what you want with your body does not mean that you get to act like a dumba$$ with my approval. Do what you want with your body when you really are 100% responsible for yourself. We give our kids a lot of freedom, and let them make a lot of choices because they typically use good judgement. We don't make a whole lot of rules, and when we do, it's always discussed as a family, so that they are part of the process and understand our motivation behind any rules. Wearing helmets is a rule in our household. You don't do it, don't expect to ever drive the scooter or the car, ever. It won't happen. You can't be trusted to be safe on your bike, you certainly aren't going to be trusted to upgrade to something even more dangerous.
So, I wouldn't keep him off his bike. I'd just make sure he understands that, when he chooses to not wear his helmet, he is showing a disregard for his safety as well as the inability to use good judgement and deal appropriately with peer pressure. He's also showing a disrespect for your wishes. These choices will be taken into consideration when he wants to get his driver's license, or when he wishes to stay out later with friends, or when he asks to do some other activity that may involve him having to rely on good judgement. His poor choices now will impact his ability to be trusted in the future, and he will have no one to blame but himself.