I will weigh in here.
First, If you change to 21, then you MUST change the military age to 21. You can't ask 18 year old boys to go fight and die for our country with the exact same weapon you also tell them they are not allowed to own because they aren't mature enough to be using that weapon (I guess unless the dead person is determined to be a bad foreign enemy guy...in which case I guess an 18 year old military boy/man not being allowed to kill a school shooter in progress is ok with you because of age). I feel the same way about alcohol. A kid can get blown up by an IED but not buy a beer? History has treated 18 year olds as adults for centuries. Yet, you want to hold them to a different standard? Once again, it is not the 18 year old boy.......it is the upbringing and parenting and expectations and accountability and discipline of that boy. People need to figure out a way to fix societal problems leading to these deranged SOB's and stop trying to legislate everything with a simple law change. Mexico eliminated personal ownership of firearms (OF ANY KIND). They confiscated any and all of them. Today, corruption is rampant from the President of the country all the way down to every mayor and every city counsel. They ALL are on the take from the drug cartels because it is either that or be killed (with the same weapons we are discussing here)......and the only people who have weapons are the drug lords, who now run the country. If you think that is far fetched and can't happen I believe there is a lot of misguidance of reality.
Second, let's get all age and ID verification equal across ALL aspects of society, and then I would agree with you. If you have to show US "Federal" Gov't "approved" ID (a new concept that Okla is yet to comply with) to buy beer, get on an airplane, get a driver's license, cash a check, check into a hotel, use a credit card (the list is actually almost endless), then you should also have to show the very same thing to vote. If anyone gets caught defrauding the US Gov't approved ID system should be deported immediately if not a US citizen and if they are, it should be a felony. THEN, I say do your background checks and waiting period......go for it.
Third, let's look at Israel, where their enemies have tried to target schools since the 60's. No school child deaths in that period. Why? The schools are heavily guarded.....AND.....in order to be in school admin (teachers AND admin), you have to be fully trained and qualified in defending the students with semi-automatic weapons. And guess what? There have been several instances where attempts were made by Palestinians on school children and they were blown away by the teachers before one single child got a scratch on them. Again, after 9-11 we changed society about "access" to airports, gov't buildings etc.....but not schools after multiple mass murders at schools. I would ask why? It's because the left would rather have a straw-man (boogie man) to argue their anti gun lobby than to ACTUALLY fix the problem. If Israel had taken this approach there would be a whole lotta dead jewish children