I know you aren't suggesting that police should have the power/responsibility to pull cyclists with helmets over, so as to check the fit?

I really don't have a problem with mandatory helmet laws for minors, especially if there's data showing these people are most likely to be in an accident, and most likely to have brain damage as a result. When this happens, it will be the tax payers who fund most of the hospital stays. A 16 year old who's transformed into a vegetable is going to cost the state quite a bit of money. Given that, it seems fair enough to assume the state has an interest in keeping down the number of hospital beds it's paying for.

Now if there's actual data that shows helmets ( as opposed to helmet laws ) don't help at all in preventing brain injuries among cyclists, I'll change my mind.

Murder laws don't prevent murder, as some of these still happen. But murder laws definitely mean there's less murder than there would be if the laws didn't exist. And laws that don't exist can't be enforced.