You make a good point Lycanthrope, but I do remember a time when boys in the second and third grade routinely recieved .22 rifles for Christmas and birthdays. When I was in high school many kids kept rifles in their cars so they could get in a little hunting on the way home. There were no school shootings back then.
Should a three year old be allowed to carry a firearm? Under my definition, that is indeed a tough one, and I am forced to agree they should not. I also agree that they should not be allowed to vote, work a full time job, or drive automobiles. The mentally incompetent and mentally retarded are allowed to vote, work full time, and drive, so why not own firearms as well? Why restrict the right to keep and bear arms based on some ethereal perception of mental capacity rather than an easily defined criteria such as age? The sad fact is, almost anyone can be declared mentally incompetent if dragged before the wrong judge at the wrong time.