According to the pilot, he attempted to start the engine while it was still warm. He noted that the starter clutch was not engaging and utilized several throttle and mixture lever positions, but the engine would not start. The pilot turned off the master switch, but inadvertently left the magneto switch in the ON position. He then exited the airplane and rotated the propeller to assist the engine start. As he walked away to re-enter the airplane, the engine started. The airplane moved forward, struck a taxiway light, and came to rest in a ditch, resulting in substantial damage to the left wing. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.