I believe that adulthood should bring certain privileges: some driver's licenses, joining the armed forces to defend my country if needed, having a beer w/ my buddy, voting for the idiots that shove this (as
@Warlock notes) Nanny State Protection down our necks, marriage & having children, among a plethora of others. I also believe that we should protect our children, to a point.
I can almost deal w/ some mandatory things for children <18: seat belts, bicycle & motorcycle helmets, limited access to alcohol (not complete abstention), just as a few examples. Again, look at most of us what, 50+? We made it alright. Bumps, bruises, the occasional broken bone, sure. That was called growing up.
I don't believe that age-related restrictions is hypocrisy. There's a reason you don't give a class 1 license to a 14 year old: They're simply not ready.
Nor do I believe that strapping a bicycle helmet onto your child's head exhibits any particular degree of common sense. Common sense would be taking your children out & teaching them how to properly ride that bicycle, turns, braking, hand signals, dirt & gravel trails, bouncing down off a curb & back up again, the rules of the road, the joys & dangers of bicycling. Then, at 18, if they decide to continue wearing a helmet, so be it. If not, fine.
Education & proper training will save far more lives than any alleged safety device.