Sorry guys, can't blame this on Millennials. The average age of mass shooters is 35 y.o. That puts them one year older than even the oldest Millennials.
This isn't a "kids these days" problem.
Perhaps there needs to be sub classes of mass shooters. That's a hell of a thing to have to say but as I think back I believe there are clearly at least 3 different motives and that explains the age difference in some ways.
First class would be the younger mental case that wants to go to his school or theater and kill a bunch of people (sometimes those he knows) to make a statement because he was bullied or feels out of place, unloved, reacting to prescription drugs, etc.
Second is the older man who was fired, cheated, or cheated on who wants to extract revenge upon those who did him wrong in any way he can, I think these mass killings have victims that are much more specific.
Third is the killer with a cause, be it religion, politics, race or other, and his age can vary, I believe we are just beginning to see the rise of this mass killer.
To me the most prolific modern killers have been the youngest of the crowd, mainly what you call Millennials I guess and I do believe they have been showered with the sense of entitlement and non judgementalism that makes the most unstable or over medicated react in the way we've seen at times when they are faced with the reality of the real world.