After last month's mass shooting at a high school in Parkland, Florida, Donald Trump said he favored raising the minimum age for buying rifles or shotguns from federally licensed dealers, currently 18, to 21. On Monday he backed away from that position, saying the decision should be left to the states.

The president was immediately criticized for kowtowing to the National Rifle Association. But there are sound reasons, aside from crass political considerations, for questioning the effectiveness and fairness of new restrictions on young adults' access to firearms.

Since the Parkland massacre was perpetrated by a 19-year-old with a Smith & Wesson M&P 15 rifle that he legally purchased from a gun store, the idea of banning such sales is superficially appealing. But there is little reason to think it would have a measurable impact on mass shootings.

Of the 23 deadliest shootings in modern U.S. history, three were perpetrated by killers younger than 21 who used rifles. In addition to Parkland, there was the 1999 attack at Columbine High School in Colorado and the 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut.

The Sandy Hook shooter, who was 20, used a Bushmaster XM-15 bought by his mother, so a higher purchase age clearly would not have thwarted him. The Columbine killers, who were both younger than 18 when they started collecting weapons, illegally obtained two shotguns, a Hi-Point 995 carbine, and an Intratec TEC-DC9 pistol through older third parties.

If the policy that Trump initially endorsed had been in place, the Parkland perpetrator would not have been allowed to buy a rifle from a gun store. But he still could have legally bought one in a private transaction, where the minimum age would have been 18.

The bill that Florida Gov. Rick Scott signed into law last week goes further, making it a felony, punishable by up to five years in prison, for anyone younger than 21 to buy any kind of firearm from anyone. Florida is now one of just three states with such a rule, which still leaves would-be mass murderers with indirect or illegal options, as illustrated by the Sandy Hook and Columbine attacks.

While the public safety benefits of banning gun purchases by 18-to-20-year-olds are doubtful, the burdens on millions of law-abiding adults are clear. At 18, the NRA notes in a lawsuit challenging Florida's new law, Americans "are considered adults for almost all purposes," and it seems anomalous to deny them the constitutional right of armed self-defense, which is also a fundamental human right.

Sen. Dianne Feinstein (D-Calif.), a longtime gun controller who wants to raise the minimum federal age for buying long guns, thinks the existing policy is the real anomaly. "Under current law," she says, "licensed gun dealers cannot sell a handgun to anyone under 21, but they are allowed to sell assault rifles like the AR-15 to anyone over 18. This policy is dangerous and makes absolutely no sense….If you can't buy a handgun or a bottle of beer, you shouldn't be able to buy an AR-15."

The AR-15, a semiautomatic that fires just once per trigger pull, is not, strictly speaking, an assault rifle, which is capable of automatic or burst fire. Furthermore, Feinstein favors giving actual assault rifles to 18-, 19-, and 20-year-olds—provided they are members of the armed forces.

Florida's law likewise makes an exception for a "servicemember," which highlights an inconsistency that is logically and morally indefensible. As the NRA notes, adults younger than 21 are considered old enough to lay down their lives in defense of the country—old enough, in fact, to be conscripted into that role whenever Congress deems it necessary.

Out of uniform, however, these same young adults not only can't legally buy a beer; under Florida's new policy, they can't legally buy firearms to defend themselves, their homes, and their families. To borrow a phrase from Feinstein, that policy is dangerous and makes absolutely no sense.

© Copyright 2018 by Creators Syndicate Inc.