I'm not a fan of holdbacks. My son played at a Trinity League school and was not a hold back. He was a solid, two-year starter, but I often wondered what it would have been like if he had been held back, from an opportunity standpoint. He essentially played with players older than him in the grade below. There was clear advantages.
In regards to it being a negative socially and academically based on Al Ko's post I'd love to see some data on that. It is nonsensically to think there would be a negative effect athletically and academically. Socially, it would make sense, to a point. But what you have to realize is that all of these holdbacks are often attending schools different than the schools they were reared in, so they aren't necessarily falling behind socially with their peer group-- they are entering school with a new peer group. Academically you are essentially repeating a grade in middle school and getting a full year to take the same math and science classes you already took. This may be boring for the student but is a clear advantage. Athletically you end up competing at the same grade level with players you are a full year older than-- that is a clear advantage.
As a freshman my son was 5'8" and 180 lbs. By his sophomore year he was 6', 220 lbs. You can't tell me that playing as a freshman in the Trinity League four inches taller and forty pounds heavier he wouldn't have had an advantage. The idea that holding someone back doesn't help them athletically flies in the face of reason.
It was well documented in the Malcolm Gladwell book Outliers that players who are older than their competition have an advantage. They are typically bigger and stronger and have hit full puberty in most cases. This leads to more playing time, which equals more reps and more attention from the coaches. Gladwell wrote about Canadian hockey players on how the NHL had a statistically improbable number of players who were born early in the year, thus often playing against players in their same age group nearly a full year younger. It is well documented. https://www.arcticicehockey.com/2008/12 ... aking.html
I don't love the idea of holdbacks, but to assert that there is a negative impact on the student is illogical.