While I understand the general sentiment I think grouping kids in a cohort from 8 till 12 is a very wide cohort.
The statistics presented have very different implications whether the kid is 12 or 8, I think.
That said, as a Dutch parent we tend to let our children grow up relatively unsupervised. At least many foreign parents are a bit startled at first.
This article helps me understand them better.
3rd grade, the year kids start at 8 and turn 9 during, is when our city schools will allow them to self dismiss. 8 seems like a reasonable benchmark age to start with.