The average age of mass shooters is 33 so what do you stop when you raise the minimum age to 21? Not much.
Elliott Rodger was 22 when he went on his rampage. His weapon of choice? Aside from the knife he used to kill his first three victims with, a 9mm handgun, legally purchased because he was over 21. Nothing would have been stopped.
Seung-Hui Cho was 23 when he shot up Virginia Tech. His weapons of choice? A .22LR and 9mm handguns. He bought them legally, even passing a background check twice (he bought them on separate occasions). Again, nothing would have been stopped.
Adam Lanza was 20 when he shot up Sandy Hook Elementary. His weapons? An AR-15, 10mm handgun, and a 9mm handgun (this is not including the .22lr rifle he killed his mother with or the 12ga cops found in his car). The only gun he was legally allowed to possess was the rifle, the other two he wasn't because he was underage but that didn't stop anything. The only thing that should have been stopped would have been the killings using those pistols.
The thing that most folks are missing from all of this: Nearly every mass shooter in the last 20-25 years had mental issues and some were even reported to the police on more than one occasion. The Uvalde shooter? He used to cut himself (even slashing his own face) and threatened to rape chicks and kill muhfuckas.
^^^ That's the real problem. Guns don't kill anyone without a person willing to use it in that manner. These crazy ass dudes gettin' guns is the problem, not the guns themselves. Wanna stop mass shootings? Deal with the mental health crisis in this country and I'd put folding money you'll see a drastic reduction in this type of shit. In the meantime, figure out a way to flag dudes like this to keep them from legally buying guns.