The portion of “repeaters” in a class seems to vary pretty widely by school. It seems to be far less common among day students so schools like Lawrenceville with larger day student populations may see less of it. Schools with large international populations or highly selective admissions see more.
As repeating or delaying kindergarten becomes far more common (especially with boys) theres already a spectrum of ages within each grade. And many who repeat 9th or 10th were young to begin with, so they don’t necessarily end up older. You and your kid often won’t know who did or didn’t repeat what grade.
It’s good to know that it’s an accepted practice with a lot of potential benefits for the right kid if yours is in a position to consider it. But if your kid is average age for their grade, appropriately mature, and well prepared for their target admissions cycle then don’t worry about it.