In Malcolm Gladwell's most recent bestseller, Outliers, he spends some time discussing the unusual distribution of birthdates among the elite Canadian hockey players.
Gladwell explains it like this (via ESPN Page 2, Dec. 8, 2008):
"In Canada, the eligibility cutoff for age-class hockey programs is Jan. 1. Canada also takes hockey really seriously, so coaches start streaming the best hockey players into elite programs, where they practice more and play more games and get better coaching, as early as 8 or 9. But who tends to be the 'best' player at age 8 or 9? The oldest, of course -- the kids born nearest the cut-off date, who can be as much as almost a year older than kids born at the other end of the cut-off date. When you are 8 years old, 10 or 11 extra months of maturity means a lot.