OK- PLEASE PAY ATTENTION. If you are an Arkansas LEO, attorney, or Dealer, please speak up. Everyone else, please settle down.
There are two DIFFERENT criteria- Federal law and State law. Federal law relates to a sale of firearms AND ammunition by a licensed person (that means a dealer) and must be met in ALL states. That says you have to be 21.
HOWEVER- state law is specific to THAT state. What applies in YOUR state does not apply in others. In many states, you must be 21 to purchase a handgun from anyone, or to possess one. That MAY not apply to Arkansas when addressing a transfer between private parties that do NOT have a FFL. There ARE states that permit private purchase at 18.
As far as having to have a permit, etc etc ad infinitum- again, those requirements are state specific. What YOU need and what the poster needs are two different things IF you are not in Arkansas.
To the Original Poster- I am NOT an attorney- am an engineer, gun collector, shooter. If I offer you legal advice, and my advice sux, you can tell the nice Deputy putting cuffs on you "Hey, some dude on the internet called C3 told me it was OK!". At that point, I'm sure he will take the cuffs off.
I would urge you to put this question to the Arkansas State Police for an accurate and current answer. I will tell you that Federal statutes will not let you buy ammo for a handgun until you are 21 (anywhere in the US) and that Arknsas DOES have restrictions on carrying a firearm- not just a concealed firearm.