Has anyone ever compared the ages of the shooters in these types of crime? Maybe it's time to recognize that maturity may not be linked to 21 years of age. Especially now a days. Maybe assault weapons should be withheld until 30 or something so you may have outgrown immaturity and someone's mental illness will have become evident. Unless you have an honorable discharge of course. Just a thought.
This basic premise falls into the trap that some how an AR-15 is fundamentally different than any other semi-automatic rifle, or that only "assault weapons" could be used to commit such atrocities. Many incidents of spree/mass violence, did not utilize "assault weapons", based on the foundational logic of your statement, it should read "Maybe all firearms should be witheld until the age of 30...".
Furthermore who's to decide what age and metal illness is appropriate to restrictions? The attack in Norway which had a much higher number of casualties than any American incident was methodically planned and executed by a 32 year old, who legally obtained the firearms and materials(albeit under false pretenses), used in the attack, in a country that has more restrictions than the US on firearms.
An exemption for honorable discharge does not mean "safe" person. Charles Whitman was honorably discharged from the Marines, Lee Harvey Oswald received a hardship discharge from the Marines, which is not a punitive discharge.
Like many of the ideas presented as of late these ideas may seem nice on the surface, but they do not pass scrutiny.