It all depends on the state, unless they are a named exclusion on the policy its still no different than anyone else driving the car. I do know that companies like Hagerty specifically state no one under 25 is allowed to operate the vehicle. Like so many other things its all in the fine print. I had several agents tell me that you are under no obligation to report new drivers in your household but if asked and you lie its trouble. I never told and they never asked.