What are you suggesting to keep your child or your grandchild from being a bloody mess on the classroom floor?
Why are you suggesting sacrificing the rights of a group of people without any proof that it would help?
Do you really think a kid that wants to do this sort of thing is going to be stopped by an age limit? This shooter was calculated and as far as I can tell did not fit our normal view of an active shooter that just wants to die at the scene. He had a plan designed to defeat the security protocol at the school, and he also had an
exit strategy! He finished shooting, evacuated with a group of students, and then went to Walmart and McDonald's. He was stopped on the way home because of the color of his shirt. If he bought a change of clothes in Walmart he would have been in the wind.
Now let's look at how the under 21 age group would be prevented from purchasing a rifle - it would be a combination of dealers and the NICS check. The NICS program is incredibly susceptible to fake IDs. Remember, it's a list of people to deny. If you're not on that list your ID can say "Micky Mouse" and you would get a green light. This is not a new issue either. Several years after the NICS system was implemented the GAO did a report on this. 100% of fake IDs resulted in a passed background check, and they haven't fixed anything since the report. To fix it would require a complete overhaul of the system.
How many fake IDs are out there? I can guarantee you that there are a good number of fakes in the high school and early college groups (mostly for buying alcohol). Do you really think someone who would go through this planning process couldn't find a fake ID?
So while this proposal would restrict a group of law abiding adults, there are plenty of holes in it for bad guys to get the guns anyway.
I'll agree with several other posters - it is disheartening to see the number of members who are willing to give away the rights of other people as long as it doesn't inconvenience themselves.