Interestingly enough, the relative age effect doesn't appear in the two other major American sports leagues. (These data on NBA and NFL players born since 1950 were provided by Sean Lahman, who has edited encyclopedias on pro football and basketball.)
The relative age effect might not be prevalent in the NFL and the NBA because size is a bigger factor in those two sports than in baseball and hockey. Since an athlete's ultimate height and weight aren't clear until fairly late in his youth, league cutoff dates aren't as important in determining one's athletic destiny. Another possibility is that (men's) basketball and football are much more popular high-school sports than baseball is. Since the cutoff date for high-school sports is more variable than that for organized youth sports, the influence of birth month in youth basketball and football leagues is relatively minor.
If you find all this data convincing, perhaps you're already planning an August birth for your little slugger. Not so fast. In 2005, USA Baseball, the nation's governing body for amateur baseball, announced it was shifting the "league age determination date" from July 31 to April 30. This change was made so the age-cutoff times more closely jibed with the baseball calendar: Under the previous rules, a player who turned 13 on July 30 would've been ineligible to play in that summer's 12-and-under league despite the fact that he would've been 12 years old for the entire season.
At first, this change was fiercely debated by the various youth baseball organizations, many of whom couldn't even agree on one date internally. It looked possible, then, that parents might be able to shop among different youth baseball organizations, blunting the impact of the relative age effect. However, this year, for the first time, all the major youth baseball organizations have fallen in line and will be using the April 30 cutoff date. Future Juan Pierres, take note: If you want to make it in the majors, forget about August. Make sure you're born in May.