This is a claim (or, rather, an hypothesis) made by Scott Adams in a recent blog post, "Parenting that Makes a Difference":
My hypothesis is that the month you conceive is the most important factor in a child's success. And no, I don't mean horoscopes are important. What matters most is how old a kid is for the class he is placed in. Macolm Gladwell described in his book Outliers how the older kids in a class are identified as gifted athletes, when in fact they are simply older. Coaches give more attention, training and resources to develop the perceived talents of older kids, thus widening the gap over their younger classmates.
Actually, Adams claims that it's "the most important factor" , not just an important factor.
From the article mentioned in Adams' post, "Outliers: The Story of Success by Malcolm Gladwell":
A close look at the rosters of top Canadian hockey teams reveals an oddly disproportionate number of players born in the first three months of the year. The reason is relative age. Canadian youth hockey leagues base a player’s eligibility on the calendar year, so skaters born on January 1 play with boys with December birthdays. At nine or ten years of age, several months can make a noticeable difference in a child’s size and coordination. The coaches then tend to label the bigger, more focused players the better ones, when in fact what they are is older. Those kids go on to get extra practice and playing time, and eventually do end up being better.