I was in a gifted program in for elementary and middle school, and now that I think about it, in 8th grade, of the top 5 academically performing kids, 4 had birthdays in January or February, and 1 was redshirted... hmmm
I wonder if their parents planned that, or if it was a result of their developmental advantage/other environmental factors/own personality/aptitude?
Anyways, fascinating idea.
For example, if your school told you your 5 year old is smart enough to start 1st grade, you'll likely enroll it into 1st grade, making it younger than the average 6 year old 1st grader.
Unfortunately this has the problem of straining the kid, and rather than seeming and being exceptional, it'll appear average. Later on in life when grades, extracurriculars, become important for college admissions, the kid will be disadvantaged because of this, since age is usually never considered.
Moral of the story: it's better to be a big fish in a small pond.
In Freakonomics, the idea is that older kids will be more physically developed, thus get more attention and skill development. Those kids will end up being better, get picked for the junior teams, who then get picked for national teams, etc. As a result, on any given professional team, you have a roster of players whose birthdays statistically favor the earlier months of the year.