I don't know about the age of the perpetrators, but
https://www.nationmaster.com/country-info/compare/United-Kingdom/United-States/Crime is interesting.
Basically the murder rate in the US is about 4 times higher than here and, unsurprisingly, the number of murders with a firearm is over 130 times higher. So I'd say that at least some of the murders committed with a gun in the US are committed with some other weapon here, but that the common ownership of guns makes murders multiple times more likely.
At the 2017 London Bridge attack, three jihadists tried to indiscriminately kill as many people as possible using knives and vehicles. Eight minutes elapsed between the first emergency call and armed officers arriving on the scene to shoot dead the perpetrators. The death toll was eight.
At the 2019 Dayton shooting, one lone nutjob tried to indiscriminately kill as many people as possible and he had a semi-automatic rifle. 32 seconds elapsed between the first shot being fired and armed officers shooting the perpetrator dead. The death toll was nine.
It's an unfortunate fact of life, that you're going to get the occasional deranged, evil psychopath. But when anyone can buy, almost unrestricted, the latest killing machine - more easily than they can buy a car - then orders of magnitude more innocents are going to continue to die in these rampages.
You're right when you say that full gun control - as we might understand it in the UK - is never going to happen in America. The second amendment, along with twisted notions of what personal freedom really means, are too deeply ingrained. As is hatred and distrust of government. But there are a million smaller common sense steps that could be taken to lessen the odds of another school mass murder happening tomorrow.
They're not going to happen though, at least on a federal level - in fact, laws might go backwards if the conservative Supreme Court rules against the state of New York's restrictive laws on the right to publicly carry later this year. And it's all because of greed and lust for power......
(although one place you won't be allowed to carry guns? The NRA convention in Houston later this week.....at least when Trump is speaking).
I am now American, but I've basically given up on the idea of a United States of America.
The federal government is impotent, so the sooner the country devolves into a loose collection of 50 independent states (as, to be honest, most of the Founding Fathers intended), and I can reside in my blue state safe in the knowledge that the laws at least somewhat align with my principles, the better.