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.
Read what they wrote again. They said guns and ammo sales to those under 21 were banned. That is false.
They didn’t say handguns and handgun ammo.