The drinking age change from 18 to 21 was pushed by Ronald Reagan and it wasn't Congress, it was Reagan. States were forced to change the laws otherwise FFederal highway dollars were to be withheld from the State(s).
There is no age limit for doing something stupid. For every 18 year old who does something stupid, there's likely 10 times as many older stupid doers. There are responsible 18 year olds and irresponsible 18 year olds just like there are responsible 50 year olds and irresponsible 50 year olds. Age is less of a factor than mentality which you've pointed out.
Again, you can't stop crazies from doing what they are intent to do. What my suggested criteria would have done is to possibly prevent those persons from buying firearms and immediately take a nutters action. That was part of the rationale for the 24 hour waiting period associated with the required background checks for purchases.
