Gyms in the US buy liability insurance on the private market, and insurers charge higher premiums to gyms that allow adults because claims for injuries are expected to be either more frequent or more expensive.
A lot of the cost has to do with the way health insurance works. If an injury can be blamed on the gym, the gym’s liability insurer and not the injured person’s health insurer has to pay the costs of treating the injury. (When there is a health insurance claim for what appears to be an accidental injury, the health insurance company contacts the patient to demand information about the incident and any potential sources of cost recovery. Our family has received several of these inquiries after fractures, etc.) In many cases this results in much higher health care costs, as the liability insurer does not have discounted rate agreements with health care providers the way a health insurer does.