While the US policies are mental, these restrictions are not one and the same, alcohol ones are largely based on the exaggerated physical and mental effects of drinking in the young where with firearms it's based on responsibility and safety which can be taught. I was educated on gun safety from 13 odd with the army cadets etc, looking back I would trust teen me with a loaded weapon a lot more than a bottle of vodka.
I feel a huge issue is the US has as a nation kind of just accepted mass/school shootings as the way it goes sometimes, unless they were involved they mostly find it hard to be particularly outraged (not all obv) Compare this to Dunblane in the UK, which pretty much stopped gun culture dead overnight, in the near 30 year since I don't think there's ever been a significant push to reverse this.