I know that an individual must be 21 to purchase a handgun and handgun ammunition from a FFL dealer. I also know that a legal guardian can purchase a handgun and ammo for an individual over 18. I cannot find any law specifically prohibiting an individual over the age of 18 and below the age of 21 to purchase a handgun from a private individual. I'm assuming that the individual fulfills all other requirements to firearm ownership such as not being convicted of a felony.
details on minimum age of purchase of a firearm in LA via smartgunlaws.com: http://goo.gl/LU0uGw
"Unlicensed persons may not sell, deliver or otherwise transfer a handgun or handgun ammunition to any person the transferor has reasonable cause to believe is under age 18, with certain exceptions" -18 U.S.C. § 922(x)(1), (5) : http://goo.gl/NkF730
In Louisiana, a private individual can sell a handgun to you as long as you are at least 18 years of age; lack of knowledge of age is not a defense. The seller also cannot sell a handgun to you if he/she has knowledge that you have been convicted of certain prior felonies, no matter what age you are. Therefore, if you have never been convicted of any felonies and are at least 18 years old, then you should have nothing to worry about.
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