You cal give/sell a long gun to an 18yo, but MUST be 21 for a handgun.
NOT TRUE AT ALL
Federal law prohibits an FFL holder from selling a handgun or handgun ammo to someone under 21 (but at least 18). There is no such restriction on personal sales.
You'll have to look to AZ state law to see if there are any state restrictions. There is no federal restriction as long as you are not a FFL holder (I doubt you'd be asking the question if you were).