This is the 'moving goal posts/impossible expectations' rebuttal. I'm sorry you can't see the entire article as I can, but I have access to gated documents, and you don't.
I can't reproduce the full article here because I'd be breaking copyright laws.
Your argument here is a non starter because while there may be methodological flaws, you're unable to point out which ones the author made (though I doubt you could, even if you could read the article). So your entire argument here is really null and void. You have to take what I'm saying at face value.
Here's what you can do: find a study that contradicts what I've posted. That's really your only move here.
How are statistics irrelevant?
I suppose you can, and are, making the emotional argument. And that's fine. But it doesn't really hold water in a debate. Nor is it a good way to decide policy...which is what we're discussing here.
Should the age be 18 or 21 because you
feel that handguns provide more of a benefit (self defense) than harm (shooting other kids/themselves) or should the age be 18 or 21 because
the data tells us that handguns are more harmful to younger people than they are beneficial.
The rational argument, which is based on statistics and deductive logic, is that handguns cause more harm than good for young people, therefore the age for handgun ownership should remain 21. You could work out a model that looks at the probability of harm caused of an 18 year old owning a handgun (P18) vs a 21 year old owning a handgun (P21). Based of the evidence that we do have, we know that P18 > P21. Unless you can prove that the probability of using a gun in self defense (Pd) exceeds P18, then the age for handgun ownership should not be lowered to age 18.
I look to solve the problem you provided in your study without government force. To find why a 20 year old is less capable than a 21 year old. Why we have mature teenagers, and immature adults. There are adults in their 30s-50s who probably shouldn't be driving a car, yet they're legally allowed to, but there are 16 year olds mature enough to drive a car, follow all the rules of the road to a t, or close to it, but are prohibited alone. I think these are the questions we should be asking instead of just accepting things the way they are and pretty much shove that conversation into the background.