With the push to ban guns for those under 21 years of age, what share of Mass Public Shootings are done by those under age 21?

Mar 14, 2018 | Featured

With the push by gun control advocates to ban those under age 21 from owning guns because of the shooting in Parkland, Florida, we decided to examine more closely the ages of mass public shooters over the last twenty years.  The current discussion is obviously motivated because the Parkland killer was 19 years old when he committed his crimes and that motivated Florida to just increase the age that people could obtain any type of gun to 21-years-old.  It is also something that is being demanded by the students who are participating in the national walkout in support of gun control that is planned for March 14, 2018.

During the years from 1998 to March 13, 2018, mass public shooters range in age from 11 to 66.  People between 11 and 20 make up about 14.2% of the total population (using the 2000 and 2010 censuses) and 18.2% of mass public shooters and 16.2% of the people killed by these mass public shooters.  If gun purchases by those under age 21 are supposed to be banned because they make up a disproportionate share of these killers, that argument applies even more forcefully for those between the ages of 21 to 25.  Indeed, this standard could be used to ban gun purchases for everyone from 16 to 45 years of age.

To show how closely mass public shooters match the total US population as a whole, note that the median age of mass public shooters is 32, and, for the US population as a whole, it is about 36 (35.3 in 2000 and 37.2 in 2010).  So while half of the mass public shooters are over 32, half of the US population is over 36.

Of the 64 mass public shootings from 1998 through March 13, 2018, ten of the sixty-four attacks were done by people under age 21.  Four of these ten attacks were done by people under age 18, and thus they were banned from being able to buy either handguns or rifles.  In all four of those attacks, the killers had a handgun.  The proposed increase in the age limit to 21 wouldn’t have changed their ability to buy a gun.  The other six were between 18 and 20 and thus banned from buying a handgun, and a handgun was among the weapons used in one of those.    Thus in only 5 of the 64 attacks (8% of the attacks) could the proposed increase in the age limit have in theory prevented them from buying a gun used in their attacks, and those attacks accounted for 5.7 percent of all the casualties (84 out of 1,461).  Given that half of these attacks occurred with weapons that the killers were already banned from buying, the question is whether these five sets of attackers who were 18 to 20 could have still obtained weapons even with the higher age limit on purchases, particularly given that these mass public shootings were planned at least six months in advance.

While this research points out that few if any mass public shootings would have been stopped by raising the age limit on gun purchases, previous peer-reviewed research showed that the previous increase in the age that people were eligible to buy a rifle in 1994 was associated with increases in murder and firearm homicides and no changes in any other type of crime rate.

Click on table to enlarge.

Here is the detailed age distribution of mass public shooters.

A similar breakdown can be made for the share of those murdered by age of the mass public shooters.  Not too surprisingly 16 to 30 years olds make up a disproportionate share of mass public shooting deaths, but they make up a much larger share of murders than they make of mass public shooting deaths (about 58% to 49%).

Some graphs can illustrate the age distribution of these mass public shooters and their share of people killed.

These numbers can be compared to the percent of murders committed by different age groups (cases where the age of the murderers is known).  One thing is quite clear: young people account for a much larger share of murders than they account for murders committed in mass public shootings.  The graph below shows the information for 2016, but very similar graphs are available for other years.

Here is the data used to create these graphs. Click on the screenshot to enlarge.

Definition of mass public shooting.

We used the traditional FBI definition of mass public shootings in all our posts on this (e.g., here, here, and here).  There are several parts to this definition.

  1. The official FBI definition of mass public shootings excludes “shootings that resulted from gang or drug violence” or that occurred in the commission of another crime such as robbery.
  2. The FBI also includes only shootings in “public places” such as: commercial areas (malls, stores and other businesses); schools and colleges; open spaces; government properties (including military bases and civilian offices); houses of worship; and healthcare facilities.
  3. From the 1980s to 2013, the original FBI definition of “mass killings” had been “four or more victims slain, in one event, in one location,” and the offender is not included in the victim count (CRS, July 30, 2015).  In 2013, the definition was changed to “three or more killings.”  Because of that academics such as James Alan Fox have also used and  continue to use this definition of four or more killed for looking at “mass shootings.”  See also studies years ago such as Grant Duwe, Tom Kovandzic, and Carl Moody, “The Impact of Right-to-Carry Concealed Firearm Laws on Mass Public Shootings,” Homicide Studies, Nov. 1, 2002.  Even groups such as Bloomberg’s Everytown have recently used the four or more definition.  But whatever the political motivations for reducing the FBI number to “three or more,” most academic have continued to use the four or more definition.



  1. Stephen

    Does the data account for the percentage of 18 to 20 year old who used a firearm that they legally purchased to commit murder?

    • Matthew Carberry

      Murder or mass murder?

      In any event, you’d have to go state by state to find the legal private sale age for handguns and rifles, for most it is 18, legal adulthood, and then go case by case within those state’s killers to see what weapon was used and if how they acquired the weapon was recorded. Noting that, if they were a prohibited person, a private purchase is illegal for them to attempt regardless of age or state law, and knowingly providing them with a firearm is a crime for the transferor.

      Per the raw data, the number of murderers using all methods (though firearms are used in the majority of homicides in the US) at 17 and under is fairly low and the majority of violent criminals even at that age are known to police even if without convictions. Per convict studies they are not usually directly buying firearms from unknowing sellers but acquiring them (lending, theft, or as payment for debt) from friends, family, and associates. A fairly high number of recovered firearms in high-crime neighborhoods have been found to be “community guns” shared within such associations.

  2. Bryan

    As far as raising the age to buy guns at 21, if 18 years of age is old enough to be drafted or enlist in the military, serve, and go to war for your country, then it should be old enough to buy and own guns.