Yeah, because the US "justice" system famously never treats juvenile defendants as adults in order to mete out draconian punishment that eliminates all chances for rehabilitation..
The US Supreme Court prevented people who committed crimes as juveniles from being executed a number of years ago. Trump’s group might change that, but for now it is the law. They technically cannot be incarcerated forever either.
That doesn't mean they can't be tried as terrorists. The main problem here is actually whether or not the facts of the crime actually allow for a terrorism charge. Fact is, he had a manifesto (see ideological goals), and the shooting was a violent criminal act.
According to the FBI that's all it takes. It may also be what is lacking in the case of some school shooters.
While I am generally on the side of "CEO FAFO", I recognize that the problem here is that the FBI and the laws they follow are flawed (probably deliberately) in such a way that they only target those who target the wealthy.
Shooting up a school is an act of terrorism if you do it because you're targeting a soft target in an attempt to hurt the local, state or federal government or you're religiously motivated etc. But not if you were bullied.
There's been plenty of over 18 mass shooters who also haven't been charged with terrorism. And with each one there's people who will say they don't want the US to become more of a police state because they believe that counterterrorism techniques (which we use internationally) shouldn't be used against the general population.
The federal government has a habit of overstepping the rights and freedoms of the general public any time they feel like they are under attack. We saw this with 9/11 and the Patriot act. So I can see their reasoning even if I don't agree that mass shooters should be considered terrorists under the law.