And as someone that played D1 sports I have pretty universally always called nonsense on that notion. I was smack in the middle of my graduating class age wise. I had guys that I played with that were old and young and it virtually never mattered nor was it ever considered (quite literally never had a single coach ask about birth date as part of the recruiting process). The most physically mature guy in my freshman class in college was far and away the youngest.
To assume that age within a draft year makes a marked difference relies on the assumption that bodies all develop relatively linear and more time = more body development (which I think we all agree is false). I was an early grower and tapped out in height at age 14. Others (even within my own family) were growing still until they were 25.
The month by month tracking for NHL draftees (literally the only sport where people even discuss or care about it as part of the draft process) is mostly filled with nonsense and I'd peg any sort of trends into more of a correlation/coincidence than age being a true causation of the outcome.
Not every sport has this problem but in sports where access to coaching and practicing is limited it is even more apparent.
When kids sign up for their first season at 6 and teams are sorted by birth year the kids who are developmentally older by as much as 10% of their life are going to be placed in groups and teams that receive better coaching. Then at 7 the kids that were on the best 6 year old team and got the better coaching will be on the better 7 year old team and receive more quality training.
There is no biological reason for this but eventually as the month age gaps become less significant the gap in quality of training is the greater reason for why those that were older and more developed at 6 are better.
I don’t want to take the time to validate the data myself but birth month is not evenly distributed for NHL players.
This is also an issue in other sports as well. Baseball has a higher percentage of players born in September/October because LL cutoff is Aug 31.
If you want more data and explanation the first chapter of the book “Outliers” is almost exclusively about this issue with Hockey and world junior rosters.