ARS 13-3102.B.3 . I reccomend reading it rather than seeking opinoins in an online forum.
However, since you broached the subject. Yes, you can open carry between 18-21, but please remember, how YOU look and behave can determine how successful future changes to Arizona's gun laws can be for 18-21 year olds.
If you are under 21, follow the law and carry like a professional. Spend money on a good holster. Dress like a geek, cover the ink and take out the shrapnel (piercings). You are a cop magnet carrying a gun. Please remember that you represent ALL under 21 gun toters when you carry.