California law, specifically Vehicle Code Section 21212, requires that every person under 18 years of age (a "minor") where a helmet when skateboarding. There is no helmet requirement for skateboarding adults.
What is the effect if a skateboarding minor is injured in an accident and was not wearing a helmet? If a minor suffers a head injury in a skateboarding accident and was not wearing a helmet, then if the minor makes an injury claim, the defendant will be able to use the minor's failure to wear a helmet against him. The defendant will argue to the jury that the skateboarder was "negligent per se," i.e., that the minor violated the law and that the violation was a substantial factor in causing the minor's injuries. The jury will then be asked to factor in the skateboarder's violation of the law in assessing how much money, if any, to award to the injury victim. The defense may argue that the jury should award nothing to the skateboarder because a helmet would have prevented any head injury. The skateboarder's lawyer's job is to lessen the impact of the helmet law violation by arguing that even if the skateboarder had worn a helmet, he/she still would have suffered a brain injury. The attorney for the skateboarder will likely need to bring in an expert to provide this testimony to the jury.
If a skateboarding minor who was not wearing a helmet does not injure his head, but suffers other injuries, then his or her failure to wear a helmet was not a substantial factor in causing his/her injuries, so negligence per se does not apply. If a helmet would not have prevented the injuries the minor suffered, then the minor's violation of the helmet law should not have a substantial impact on the value of the claim. The other side will try to portray the skateboarder as irresponsible for not wearing a helmet, but if a helmet would not have prevented the injuries suffered, there is a strong argument that the minor's helmet-less status is really irrelevant.
What is the effect if a helmet-less adult suffers a head injury while skateboarding? Since there is no helmet requirement for adults, the adult was not "negligent per se." However, the other side will certainly argue to the jury that a helmet would have prevented the brain injury, and that the jury should factor that into any award to the injury victim.
Bottom Line for Minors: If a helmet-less minor suffers a head injury while skateboarding, you will want to hire a lawyer who is experienced in handling similar cases and who is willing to spend the money to hire an expert who can explain to the jury that a helmet would not have prevented a head injury.