If it’s designed to address the cognitive development issues, then 21 isn’t old enough.
If it’s not, then what purpose does it serve, other than to entrench the notion that 18-, 19-, and 20-year-olds are not really adults?
That kind of limbo is one factor in marking the 21st birthday, since the journey to adulthood is a long one marked by many milestones in a population eager to get there. That’s fine with me, but currently the main symbol of 21 is drinking, and one way of celebrating is overindulgence. I’d hate for guns to become a part of that.
I also think of the difficulty under-21s have of acquiring alcohol, which is… very little. There’s a lot of smart people with a lot of time on their hands.
Also, MOST mass shooters are over 21. The factoids promoting this restriction are full of qualifiers, basically saying that (1) most of the (2) recent (3) high-body-count shooting events involved men 21 and under. But when I go searching for who kills people with guns, the average age rises considerably when you take into account all the bodies involved.
I think one is on a slippery slope guessing at neurodevelopment. If there is evidence shootings and other tragedies tend to involve people age 18-21 (or whatever), perhaps it may make sense to restrict what is available at that age. If there is no evidence, a restriction would not help, and since I prefer small government would like as few restrictions as needed to reduce problems. If restrictions do not do anything they serve no legitimate purpose.