The age of majority in Florida is 18. Licensed dealers can sell long guns to anyone 18 and older and handguns to anyone 21 and older, assuming they're otherwise not prohibited.
Unlicensed, private individuals can sell rifles and handguns to anyone 18 and older, assuming they're not otherwise prohibited.
Licensed Florida gun dealers (FFLs) are governed by federal law as are all licensed gun dealers. The applicable code is in Title 18, Section 922 of the US Code, as follows. The Code does not concen itself with private sales between unlicensed individuals.
As always, regardless of what any particular state statute says or doesn't say, the final determination as to whether a seller will sell a particular firearm to a buyer is up to the seller.18USC922
TITLE 18--CRIMES AND CRIMINAL PROCEDURE
Sec. 922. Unlawful acts
(b) It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver--
(1) any firearm or ammunition to any individual who the licensee knows or has reasonable cause to believe is less than eighteen years of age, and, if the firearm, or ammunition is other than a shotgun or rifle [ie, a handgun], or ammunition for a shotgun or rifle [ie, a handgun], to any individual who the licensee knows or has reasonable cause to believe is less than twenty-one years of age;