The problem is that kids may choose not to wear a helmet and therefore not to ride. With younger children I think it is a good idea to wear a helmet as their skulls are thinner and they have more low speed crashes - but at that age their parents are likely to very influential in their helmet wearing or not. Once you are talking about teenagers they are perhaps the group that should be cycling from a trying to keep them active, freedom etc but they are also the group that is most image concious.
But using your seat belt analogy I am worried that the seat belt rules changed ... when my eldest was pre 5 she could move out of a seat when she was 3, then it changed for my second child to 6, and by the time we were onto the third child they changed the rules so that the middle child had theoretically to go back into a child seat until she was 10. Is it the start of compulsion for everyone?
I also think in the same way that seat belt rules (and helmet wearing) amongst children are much higher in middle class areas, than in the council estate areas and that the same affect would be seen in helmet wearing.
I wear a helmet myself, but I don't want to see compulsion for anyone.