i think arguing over it is pointless because whether you like it or not; kids will always find ways around your authority or wants to do the dirty.
although personally, if the child is uniformed i'd have to say....18? at least at this point they are old enough to get a full time job to support whatever happens and should already be out of highschool.
if the kids informed of the risk and is blatantly clear with the need for protection, then hell. let them have at it whenever they want just as long as they know how imperative it is that they do actually wear/use protection and actually follow through with it.