Justice "John Paul Stevens: Repeal the Second Amendment"
— and raise the age for purchasing guns to 21.
I'll repeat (and keep repeating) my objection to raising the age for gun purchase to 18, as part of my objection to all rules outside of Constitutionally-mandated ages for public office that undermine 18 as the age of majority.
For mass shootings in the US generally, the shooters have been almost always 20-49-year-old White males. Youngsters are an issue only with mass shootings in schools, which are just under a quarter of mass shootings in the US, which in turn are only a small portion of gun deaths.
A campaign for eventual repeal of the 2nd Amendment is a good idea if, but only if, it helps and does not hurt immediate efforts to ban civilian possession of semi-automatic weapons outside of shooting ranges — plus restrictions on ammunition-magazine sizes, bump stocks, conversion kits to make semi-automatic weapons automatic, large-quantity purchases of guns and ammo, etc.
Meanwhile, we need to reinforce vigorously 18 as the age of majority with full rights and responsibilities, working against trends toward arrested development. We also need a lot of research and other efforts to make adulthood in the US suck less.
Yet again, no, The Kids Are Not Alright (sic), but in terms of social pathology they're usually doing better than their elders. To repeat Mike Males's points, except when they're doing better, older US teens are a normal adult US population. And for the US population generally — especially older White males — we need to work on the epidemics of alienation, loneliness, anger, rage, and despair.