Your statement about those under 18 is not correct, near as I can tell. Except for very limited circumstances (training, supervised target practice, etc.), those under 18 cannot possess a handgun at all. From 18 to 21 it's probably what you said above; can be possessed, but must be acquired by a legal method NOT including a Federally licensed dealer. From 21 up, a person can buy from the dealer or any other legal source.
Again, get legal advice from lawyer-type folks if you intend to act upon it. The local police, state police, or county sheriff can also be good resources for info on guns. They can also be as clueless as any other anonymous Internet keyboard commando, too, so consider yourself warned.