Who Is likely to Perpetrate a Mass Shooting?

I haven't seen any data on the subject but running my mind back over the litany of mindless mass shootings over the last couple decades, it seems like every one of the shooters was a young man (I don't recall if the wife of the San Bernardino shooter took part in the actual shooting).

Given the difficulty we have as a nation coming to grips with any kind of effort to keep guns out of the hands of possible terrorists perhaps it would help to focus on identifying the characteristics of those who actually do the shooting.

Criminal statistics have shown for years that most violent crimes are perpetrated by young men, mostly between ages 18 and 25, perhaps that is where we should start with limiting access to killing tools.  Perhaps people under the age of, say, 30, can't buy a weapon without going through some rigorous process of training - sort of like licensing drivers.  We would also have to make it a crime for anyone to sell or give weapons to persons of that age to avoid having them just get someone else to buy one for them.