@fretfulmother ,. Really, it depends. The schools want students to succeed and they know their environment. And they can fill beds and seats without duping people into an extra year. I don’t think their advice is as biased as you suggest. The school’s that do this most seem to be the most selective, and they have acceptance rates of under 20%.
I think that when a school does this a lot (as several do), much the way NYC private schools encourage kids to wait a year to start kindergarten, it can become a self-fulfilling prophecy. You will be young at the “right” age because everyone else is older.
There are issues for foreign students who often repeat to get their English up to snuff. Athletes may repeat for an additional year of growth and strength (which they also do as PG but that bothers people less.)
DS didn’t repeat but coming from a “good” public system, he was 1-2 years behind many classmates coming from private schools in math, for example.
I suspect that the reason for the repeat matters most. Personally, I don’t like this practice, for kindergarten or high school, so didn’t do it for my kid (with a summer birthday) at any point. But for some, it could make sense. If you’re not ivy material, an extra year of prep school isn’t going to change that but it could make you better prepared.
OP, I don’t think it hurts in many cases, but ask yourself this - if it did, would I do it anyway for the 4 year experience at one of these schools? If you can legitimately answer yes, you have your answer. BS is about a lot more than college admissions.