I think it's a start and put forth in the right way it might. 18, 21, 30, it really doesn't matter the age. Education before possession, licensed and registered ownership, periodic training (yearly class? every five years?), etc. There are lots of ways to control the flow of guns into society and even who has access to them. Yes, criminals will still be able to get guns and those that are willing to plan long term to cause some sort of massive event will slip through the cracks, but putting up barriers combined with more mental health support (in school and out) will go a long way.