“...dating back to 1968, that make it illegal for firearms dealers to sell guns or ammunition to anyone under 21...”
There is no federal law against selling guns/ammo to those under 21.
I believe that federal law currently prohibits the sale of handguns or ammo for them to anyone under 21.
Yes, there is. It is the Gun Control Act of 1968, as codified in Chapter 44 of Title 18 of the United States Code.
The ammunition is prohibited only if it is considered "handgun ammunition" which most calibers (thanks to AR & AK pistols and "The Judge") of rifle and shotgun ammo are.
The FFL cannot sell a handgun to a person under 21. Under-21 Americans can buy a handgun from a private party, though. No mention of ammo.