A pair of mass shootings perpetrated by disturbed young gunmen — one in Buffalo, the other in Uvalde, Texas — come in haunting succession. The repetition, the needless loss of life, the political gridlock easily give way to an exasperation bordering on fatalism — a feeling that nothing can be done.

Rampage homicides are rare, accounting for a relatively small fraction of overall gun homicides, and the research base is correspondingly less clear regarding what best prevents such atrocities. But available evidence does suggest the value of one commonsense measure: raising the minimum age at which people can buy firearms to 21.

Two simple overlapping realities argue for raising the legal purchasing age. First, most firearm violence and gun homicides are committed by relatively young people, with homicide risk peaking between the ages of 18 and 24. This is conspicuously true of accused, convicted or slain mass shooters:

• Salvador Ramos, 18, in Uvalde

• Payton Gendron, 18, charged in Buffalo

• Nikolas Cruz, 19, in Parkland, Florida

• Adam Lanza, 20, in Newtown, Connecticut

• Dylann Roof, 21, in Charleston, South Carolina

• Robert Aaron Long, 21, in Atlanta

• Elliot Rodger, 22, in Isla Vista, California

Experts offer varying explanations for the relative youth of so many violent gun offenders. Some experts speculate that such violent behaviour is related to ongoing brain development, especially of domains regulating impulse control, judgment and long-term planning. Others note the increasing salience of gender and identity during adolescence — a period that by some estimates extends into the mid-twenties — coupled with a preponderance of narratives linking firearm violence to masculinity.

In 2012, shortly before Adam Lanza used a Bushmaster XM15 semiautomatic rifle to kill 26 children and adults at Sandy Hook Elementary School, the firearm appeared in a series of advertisements that enticed prospective buyers to reclaim their “man card” by purchasing the weapon. Ten years later, Payton Gendron allegedly evidenced the same pseudo-machismo in online postings in which he contrasted his own behaviour in purchasing and modifying a Bushmaster XM15 to what he described as “cucked” New York State gun laws and the “cucks” who obey them — deploying a misogynist put-down popular among far-right extremists, referring to men whose wives have sex with other men. Pooled data on violent offenders and other individuals who indicate the potential for violence show that a striking proportion reveal a desire to claim masculine identities through their acquisition and use of firearms.

The second reality arguing for higher minimum ages reflects a central irony and gun policy dilemma: while young people are arguably the most dangerous — the most likely to commit a future crime — they are by definition least likely to have committed a crime in the past. They haven’t been alive long enough to create a specific, legally actionable paper trail that could be cited by medical or legal authorities after a background check to deny them access to lethal weaponry. What is more, the psychiatric disorders that most often disqualify someone from gun ownership typically first present or worsen during late adolescence and early adulthood. When they manifest themselves, it may be too late.

This problem is fundamental. It admits no clever or technical fix. No red-flag law, no fancy artificial-intelligence algorithm, will allow us to specifically identify most young people who pose serious risks to themselves or to others. We certainly wish there were. We and many of our colleagues expend millions of dollars on predictive analytics applying such methods to adult offenders and other adults with behavioural health challenges. That’s hard enough.

After an atrocity occurs, there will always be ostensibly obvious red flags. There reportedly were in Buffalo. There were in Isla Vista. With rare exceptions, though, these warning signs will go undetected before an atrocity occurs. Even when detected, they are rarely feasible as a basis to bar a specific individual from possessing a gun. After all, hundreds of thousands of teens have done disturbing things that never result in an arrest, let alone criminal convictions. Hundreds of thousands post offensive, racist or scary material on social media. None of this typically constitutes a legal reason for denying them access to guns.

More stringent background checks and mental health restrictions — such as those in the 2013 Manchin-Toomey gun Bill — poll well among both Republicans and Democrats. While such measures are reasonable, we should be realistic about what these can achieve in the case of adolescents and young adults, who simply have very short backgrounds.

Here’s a simpler, better red flag: a teenager is trying to buy a gun. Because so many serious problems arise or worsen in adolescence and early adulthood, we should shift the burden of proof away from police or others seeking to provide specific reasons for why a youth is dangerous to self or others — and make a blanket judgment that people under 21 should not be able to buy or possess these weapons.

Federal law makes people wait until they are 21 to buy a handgun from a federally licensed dealer, although they can — as Ramos and Gendron did — buy long guns at 18. And the federal laws do not apply to sales by private parties. Some states have set their own limits for all gun purchases (18 is the typical choice) and their other regulations vary widely. Some states allow youths as young as 16 to possess both handguns and long guns (or even 14, with a hunting licence, for instance).

If a national minimum of 21 proves infeasible, and some states wish to continue to sell guns to 18-year-olds, these states should explore approaches such as gun licensing and permit-to-purchase rules, parental consent and supervision for teenage gun purchasers. Such rules may be appropriate for people who are 21, as well.

Minimum-age requirements and related measures cannot prevent all mass shootings. They may not reduce more mundane gun crimes, which are so often perpetrated with guns purchased through illegal markets. That’s not the standard. Such measures will help prevent mass homicides by the next Salvador Ramos, who destroyed so many lives, including his own, because he was able to legally buy lethal weaponry.

What’s the rush to allow young people unfettered access to such weapons? We ask our children to wait a bit before they can buy a can of beer or grab a rental car at the airport. We should ask the same when they seek access to semiautomatic guns.

• Nathaniel J. Glasser is an internist, paediatrician and health services researcher at the University of Chicago. Harold Pollack is the Helen Ross professor at the Crown Family School of Social Work, Policy, and Practice, University of Chicago