A teen visiting in Lake Sherwood in southern Warren County has died from injuries he received in a skateboarding accident that happened Friday, Warren County Sheriff Kevin Harrison said.Riley Severns, 15, of Rochester, Ill., was skating down a hill into the parking lot of the clubhouse at Lake Sherwood when he lost control of his skateboard and fell, striking his head on the pavement, Harrison said.Lifeguards who were nearby performed CPR on Feverns until emergency personnel from Marthasville Ambulance District arrived on the scene.Marthasville EMS crew members called an ARCH air ambulance, and Severns was airlifted to Mercy Hospital St. Louis. He died Saturday, Warren County Coroner Roger Mauzy said. Mauzy said an autopsy is being performed to determine the exact cause of death, but the results were not available when The Record went to press.However, Severns was not wearing a helmet at the time of the accident.Mauzy said he can’t be sure if a helmet would have prevented the tragedy, but stressed the importance of wearing one when skateboarding, bicycling, or riding a motorcycle or ATV.“You can’t say enough about wearing a helmet,” he said. “There is no excuse. They really should be used. This is just one example of where a helmet might have made a difference.”