A small sample and extremely anecdotal, but, the vast majority of kids from my son's "age group" in LL who are now Sophomores (my kid is a Freshman) are extremely mediocre players now. They benefited from being bigger at ages 10-12 and seemingly bought into being told how good they were.
Keep in mind we aren't in a Baseball hotbed and kids don't have to work hard to get spots on Varsity. But being on the May through Summer side of the May 1st birthday cutoff definitely did not benefit most of the kids who in 6th Grade were competing against 5th graders.
Right. But it sounds like they benefited from the relative age effect when they were on the older side (say, a May birthday in little league) but now they are suffering from the relative age effect because they are on the younger side (i.e., their parents didn't hold them back, so they are currently 15 in March of sophomore year). In other words, they would be less mediocre as freshmen than they are as sophomores.