You think that if the law changed from 18 to 21 that would prevent the killing of people?
Do you think that person could steal a rifle from someone and use it?
Do you think that person could use something else to kill people with?
Again, do you really think the law is going to stop someone from breaking the law?
Why are you focused on the AR-15? Why not focus on nails, plumbing pipes and end caps, knives, SUV's?
It's already illegal to kill people. Restricting tools won't stop people from breaking the law. People could focus on raising their kids with ethics, morals and common sense though.