For riding on roads, especially for kids under 16 it should be compulsory, not just for your your own safety but if your messing around and go into the side of a car or something and die, the driver then has to deal with playing some part in your death even though it may be completely your fault.
I learnt the hard way how important it is to wear a lid and its defiantly worth it.
Thing is though , if they did make it compulsory you know there would be some dick heads who would stop wearing one just because they don't like being told what to do.