It is common in my affluent area, but it does vary by school and by district. It’s also common for families to skip a kid ahead a grade because they aren’t being challenged in their current grade.
D has a November birthday and we had her start kindergarten at 5 and a half. I think it was best for her. In the district we were in at the time, redshirting was very common and she would’ve been quite a bit younger then the rest of the class if she started Kindergarten at 4 turning 5. She would’ve hated always being the youngest.
For S, his birthday usually falls on the first day of spring, so the whole redshirting didn’t apply to him…and they had moved the cutoff date which meant that it was harder to enter Kindergarten if you hadn’t turned 5 yet.