Kids and helmets make sense. The helmets work well, and kids are forever falling off their bikes and hitting their noggins on the sidewalk. Make a law for them, and leave me alone. The same statistics that show kids as having the bulk of head injuries, also show that competent adults have very few accidents indeed. We have all sorts of laws based on age.
Statistics show that many adults actually ride less or stop riding because of helmet laws.
I think that there are 3 good reasons why there should only be one law one for all. We teach our kids by example. I'm sure that every adult bicycle rider has fallen off their bicycle. How do we teach kids that subtle distinction between a kid falling every week or so , and an adult every year or so. Falling off your bicycle is an accident, and we don't always control our environment enough to ensure that we don't have accidents.