All of them that have highly competitive highschool football.
If you hold them back a year in gradeschool by the time they get to highschool they are a year older than they would otherwise be ~ 17 rather than 18, or 18 rather than 19.
Some states have actually debated new laws to prohibit the practice.
There's no scientific evidence that repeating 3rd grade makes you able to read better than not repeating 3rd grade.
That would be hard to prove conclusively. It still seems to me to be the most efficient way of reviewing failed subjects. Of course, you have to understand that I came from a rural school in the sixties where, perhaps, one child needed to be held back in a given year.
When we're talking 1/4 to 1/2 of the class; other problems are at work.