Unfortunately. I am all for common sense gun laws (and there are many things we can do). But it has to: 1. Work 2. Affect the people committing the crimes (who are by far the ones committing the crimes) Simply making things illegal doesn't do anything. Murder is already illegal. Someone intent on committing murder isn't going to be stopped by a law.
In the USA you cannot buy alcohol until 21. I would not be opposed to making the legal age for purchasing any firearm to be 21. I don't think it violates the constitution because the constitution and bill of rights was written to apply to adult citizens. And it doesn't mean a minor could not possess a firearm, Daddy can still loan you his shotgun but you cannot legally purchase until 21. Obviously the military is an exception for both alcohol and firearms. I had access to both in the USAF at 17 but that is much different.
They don't allow a juvenile mental history to be used for background checks Maybe one reason these shooters are so young is because they don't have a mental/medical adult history and can still buy guns When they are older most eventually get a criminal /medical history that might keep them from buying a gun