Arizona has laws regulating the use and possession of deadly weapons defined as 'anything designed for lethal use' (ARS 13-105). Although a Court would ultimately decide on whether brass knuckles meet this definition, you would have a hard time trying to explain that your brass knuckles were for something other than smashing in someone's face.

If you carry a weapon without a permit as allowed under (ARS 13-3112) you are breaking the law.  Minors are prohibited from carrying concealed weapons.