What you would be doing there is attacking the law as denying equal protection under the laws due to the age of the plaintiff (being 18-20). That's going to probably lose. Hard. All the gov. has to prove is a rational basis for the law.....and that basis can be very, very, very, loosely tied to the law.
If we know that anything
is off the table for 2A protection for adults, it is the rational basis test.