By law, anyone under the age of 18 must wear a helmet when riding a bicycle on a public roadway, but wearing a helmet is a good idea at any age. Bicycle helmets can reduce the the severity of head trauma significantly and many bicycle helmet manufacturers offer helmets with MIPS (multiple impact protection system) technology that can help reduce head trauma in the event of a collision.
Choosing not to wear a helmet cannot be used against you, but insurance companies certainly will try. I have had insurance adjusters argue that they should pay less for a head injury because the cyclist wasn’t wearing a helmet. This is legally inaccurate because you cannot be punished for something you were not legally required to do. A helmet is not a panacea and the truth is that head injuries do happen even when riders are helmeted.