You aren't buying a handgun from a licensed dealer until you are age 21 or older. That is a federal law. Your respective state's may allow ownership and possession of a handgun by someone under the age of 21, but you'll have to find a private party seller, not a dealer. I'm not in Arizona or Missouri, so I can't speak to their laws.
But federally, an FFL is prohibited from selling anything but a rifle or shotgun to persons 18-21.
http://www.atf.gov/firearms/faq/lice...e-requirements
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Scott
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