Valid point. However, I don't think the handgun law was ever challenged all the way up to the Supreme Court. I could be mistaken.
Yes, you are correct. The following is from a Feb. 26, 2018 article on the New Republic site
"America’s regulations on gun purchases don’t make much sense no matter how you feel about the Second Amendment. By federal law, only people 21 and older can buy a handgun from a licensed dealer, but 18-year-olds can buy shotguns and rifles—including the infamous AR-15.
In 2012, the National Rifle Association implored the U.S. Supreme Court to fix this inconsistency—by allowing people under 21 to buy handguns. In its petition, the gun-rights group told the justices that young Americans should not be deprived of their constitutional right to buy a specific class of firearms. “It is undisputed that 18-to-20-year-olds were not only permitted but required to keep and bear arms when the Second Amendment was ratified,” the NRA’s lawyers wrote. A group of conservative attorneys general from 22 states filed a brief backing the NRA’s petition.
For whatever reason, in what has become a trend on gun cases, the court decided not to accept the challenge."