AZ Purchasing Laws

Okay... So I was hoping to get some clarification on Arizona purchasing laws before I agree to a favor.

I'm 22... A good buddy of mine is 19. This kid lived with me for a few months, got himself a house... I've known him for about two years.

I got him over his fear of guns.. tought him how to shoot... and happen to know that he's got a good head on his shoulders and knowlege enough to carry.

So he takes me over to a gun shop, Friday, and shows me the Beretta he wants to buy. "Sorry bud.. gotta be 21 to purchase a handgun."

Well at that point the guy work'n at the shop tells me that even though my buddy can't actually purchase the gun.. I could purchase it for him and legally 'give' him the gun; at which point he could carry, use, whatever.

So my question is: IS THIS TRUE? I know he's a safe and responsible guy... I've spent all this time preaching to him that he has a right and obligation to protect himself with a firearm... But can i LEGALLY purchase a gun for him? And how does the transaction work when I give him the gun, as far as registration and all that good stuff?

Thanks