I would send him a formal letter stating why the drivers had to be 16yoa. (I assume insurance reasons) tell him he cannot run until he's 16 and will have to prove it then. Next, I would explain to everyone else running that they must have a valid ID issued by the state proving their age so this does not happen again. Require them to give you a photocopy. This is to protect the organization. If nothing happened through the season and he got away with it, let it go. Just step up your enforcement. This was really a mistake of the organization by not covering your a__. CYA, one of lifes lessons that everyone learns eventually.