First, generally laws are written to make something illegal. In the absence of a law making something illegal it is legal.
There is Federal law making it illegal for an FFL (Federal Firearms Licensee) to sell or transfer a handgun to anyone under 21 year of age. There is no federal law making it illegal for anyone not under 18years of age to own, possess, received, or carry a handgun provided the individual is not prohibited due to other NON AGE RELATED FACTORS.
There is NO AZ law making it illegal for an 18 year old or older to possess, own, or carry a firearm.
Go back to your LEO friend and make him show you the statute that makes it illegal for a person 18 years or older who is not otherwise prohibited to own/carry/possess a handgun.... the ONLY thing he will be able to show you is the prohibition for an FFL to transfer to one under 21.
No AZ law prohibits the PRIVATE transfer, sale, gift, or inheritance to one 18 years or older who is not otherwise prohibited!
Basically, you can't buy from an FFL or have someone from another State give you a handgun (requires an FFL transfer).
And the link that GWBiker provided you HAS A LINK TO THE AZ STATUTES.
Best solution for an AUTHORITATIVE answer would be to consult an attorney licensed to practice law in your State!