Yes, 18-21 years old, OC is legal, and in fact is the only way for you to carry, unless on your own property, home or business (ARS 13-3102). Must be 21 to get CCW.
If you don't have it already, you should get a copy of Alan Korwin's Arizona Gun Owner's Guide. Available here: www.gunlaws.com, or at most gun shops. Also read the ARS concerning gun laws: ARS 4, 8, 9, 11,12, 13, 15, 17, 26, 28, 32, 33, 36, 41.
Get an inexpensive voice recorder (many MP3 players have a recording feature), useful for legal encounters and school work.
Look at the Arizona Citizens Defense League website, consider becoming a member: www.azcdl.corg.
Welcome to Arizona and OCDO! (For the record, I am in the USAF and only moved to AZ last August. One year anniversary coming up!!)