Raising gun purchase age to 21 is the laziest way of handling the mass shooter crisis
It's just an easy way for politicians to get a law passed and look like they are taking a stance on the issue. The overwhelming majority of mass shooters are over 21. The median age is somewhere around 34. The Las Vegas shooter was 62. How will raising the age to 21 stop people like him?
Also, the second amendment either guarantees your right to own a gun or it does not. If it is okay to take that right away from adults aged 18-21 then it is okay to take it away from other adults too.
Note: I'm not advocating for any particular gun reform. I'm just saying that scapegoating adults aged 18-21 is a lazy and ineffective solution.