The suggestion is the younger end of age group guys have (after 8 or 10 years of it) formed a habit out of being the quieter/smaller/less developed one of the group by the time they hit 18.
Not sure if that holds up to scrutiny or not.
The fact the stats come from major league baseball, though, certainly make me more inclined to believe it. MLB development is certainly not like football. In most cases you get recruited from high school at 17-18 to go to college. Then after 1-4 years of college you get drafted (they do 40 rounds of draft picks). Then you play rookie league. If you’re any good at that level, you play short season A level. Then if you’re any good, you play single A. Then double A. Then triple A. If you go well there, you get a shot at the 25 strong major league roster.
It’s a huge, industrial scale process of whittling and funnelling, and the failure rate is bewildering.
All of which is to say, the stats they can produce on who is more or less likely to make it is extraordinary, especially compared to AFL where there’s ~80 new guys added in the whole league every year, and you’re half a chance of playing from round 1.
Maybe we should ask Professor WOB.