I'll field this one as I've done some looking into it. If you look at mass shootings and particularly school shootings ages generally run from 15 to 19ish. There are some 21 year olds mixed in but the trend heads down after 18. . . So some data supports this. But the counter to that is all those under 18 who are getting firearms from someone else! 21 isn't a magic number, it is merely another age out there used for things like drinking, Mary Jane, etc. So it's convenient to use!
Table is age of school shooter for school shootings from 1970 to June 2020.
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