You can't sell guns where they sell alcohol, though. So, that kid can get pissed off, walk across the street to the gun store (Wait a few days...)
It doesn't have to be an Assault Rifle. It doesn't have to be a machinegun, even a Brady Ban 10 round semi-automatic (I'm gonna say .40 calibers as it's a child of that era) can cause mass terror, and give that kid a sense of Power.
Okay, the minimum age? He can vote, and he can Enlist. If you're military, and under 25, it's not real hard to get a rental agreement, but if you take away their guns, you just have a proportionate rise in stabbings, arson,
ax murders ... That same kid can go into Dollar Tree to buy Chlorox, and Windex. Threaded pipes, and matchheads. For me, it was aluminum shavings from machineshop, and rust. It's not rocket surgery, people are incredibly easy to kill.
"Oh, but he didn't kill that many people." I've actually fielded this argument, seriously.
How many is a rampage? Enough? Enough.
We invented guns. Not America, but we can argue that we perfected them to a degree, (And snub the Nazis of a lot of credit there.) Coming up with more creative ways to kill each other defines Humanity, throughout our history. Open a history book, at random, and tell me which War, or Battle it's about, it's what we do.
Guns didn't make us evil, taking them away isn't going to make us any better.