Not that AZ is that great of a state (see some other unrelated posts), but I did dig this up from the AZ Fish and Game Dept's site in response to a recent 14 year-olds death on an ATV. (There was a death just last week here in Yuma by a 10 year old on a quad)
http://www.azgfd.gov/artman/publish/article_717.shtml
I think that the typical desert rat/white trash Arizona parent will bitch and moan over the law between beers, but it does allow their kids to ride with safety equipment on. It's one thing to ride without a helmet being an adult, but under 18, if its a law, at least the few who die each year in AZ might not have died in vain. Besides, motocross helmets look really cool!