One of the bizarre omissions in the bipartisan deal on gun control was the failure to raise the age to buy an assault weapon from 18 to 21. It is incomprehensible.

The Washington Post reported on studies that catalogue the sex and age of mass shooters. The killers are almost entirely male, and a large percentage are under 21. This leads the author of the study to propose raising the minimum age for buying a weapon to 21. Curiously, there is already a federal law banning the sale of handguns to anyone under 21, but no law banning the sale of long guns for that age. So the killer in Texas could legally buy two AR15s on his 18th birthday, but was not able to buy a handgun.

When Vanderbilt University psychiatrist Jonathan Metzl learned that the perpetrator of the Uvalde, Tex., school massacre was a young man barely out of adolescence, it was hard not to think about the peculiarities of the maturing male brain.

Salvador Rolando Ramos had just turned 18, eerily close in age to Nikolas Cruz, who had been 19 when he shot up a school in Parkland, Fla. And to Adam Lanza, 20, when he did the same in Newtown, Conn. To Seung-Hui Cho, 23, at Virginia Tech. And to Eric Harris, 18, and Dylan Klebold, 17, in Columbine, Colo.

Teen and young adult males have long stood out from other subgroups for their impulsive behavior. They are far more reckless and prone to violence than their counterparts in other age groups, and their leading causes of death include fights, accidents, driving too fast, or, as Metzl put it, “other impulsive kinds of acts.”

“There’s a lot of research about how their brains are not fully developed in terms of regulation,” he said. Perhaps most significantly, studies show, the prefrontal cortex, which is critical to understanding the consequences of one’s actions and controlling impulses, does not fully develop until about age 25. In that context, Metzl said, a shooting “certainly feels like another kind of performance of young masculinity.”

In coming weeks and months, investigators will dissect Ramos’s life to try to figure out what led him to that horrific moment at 11:40 a.m. Tuesday, May 24 when he opened fire on a classroom full of 9- and-10-year-olds at Robb Elementary School.

Although clear answers are unlikely, the patterns that have emerged about mass shooters in the growing databases, school reports, medical notes and interview transcripts show a disturbing confluence between angry young men, easy access to weapons and reinforcement of violence by social media.

Federal law requires people buying handguns from licensed dealers to be at least 21. But in Texas and in most other states, 18-year-olds can purchase what are known as long guns, which include assault rifles. In a prime-time address Thursday night, President Biden called for banning assault weapons but said that, if that’s not possible, lawmakers should raise the age to purchase such a weapon to 21. “The issue we face is one of conscience and common sense,” the president said.

In the wake of the 2018 Parkland shooting and other violent acts by young men, six states, including Florida, did raise the purchasing age for long guns to 21, over the objections of the National Rifle Association. The NRA calls such restrictions a “categorical burden” on the right to keep and bear arms, while Florida state attorneys argue that because “18-to-20-year-olds are uniquely likely to engage in impulsive, emotional, and risky behaviors that offer immediate or short-term rewards, drawing the line for legal purchase of firearms at 21 is a reasonable method of addressing the Legislature’s public safety concerns…”

“Age is the untold story of all this stuff,” said Metzl, who is also a sociologist. “I feel very strongly we should not have people 18 to 21 with guns.”

The United States is one of the only countries in the world where mass public shootings are a regular occurrence. Researchers Jillian Peterson from Hamline University and James Densley from Metropolitan State University, both in St. Paul, Minn., have spent their careers tracking these events, and their research shows that attacks are overwhelmingly carried out by men whose ages are strikingly clustered around two key periods in their lives.

Workplace attacks have been mostly carried out by men in middle age. School shootings, on the other hand, involve perpetrators mostly in their late teens or early 20s. Men in these same two age groups, Peterson points out, also have higher rates of suicide largely using firearms.

A Washington Post analysis of 196 mass public shootings in which four or more people were killed since 1966 shows that nearly 98 percent, or all but five, of the perpetrators were men. Forty percent of the shooters were between the ages of 18 and 29 and another third were between 30 and 45.

Based on statistics, I’d say that the best way to protect the public is to ban the sale of assault weapons and all other automatic or semi-automatic weapons. All such weapons now in private hands should be bought back and either destroyed or sent to Ukraine.