In his book Outliers, Malcolm Gladwell discusses the odd distribution of birth months among NHL players. Because youth players are registered in leagues based on their year of birth, the biggest and strongest players tend to be those born in the first few months of the year. This selection process starts as early as age 8, and the effect persists more than a decade later in junior hockey in Canada. The effect has been visible for decades:

One thing that's very unintuitive about this effect is that, other things being equal, if you have a 17-year-old player who puts up the same number of points as an 18-year-old player, the 17-year-old will have a much higher performance ceiling. I like to call this the 'Wayne Gretzky-Dan Hodgson' effect - two players who had identical stats their last year in junior; but Gretzky was 16 and Hodgson was 19, and so it was obvious who would have the better NHL career.

At any rate, if you have identical junior players born in January and December, the December player was almost a year younger when he achieved his performance. If we project that performance forward to Age 23, then we'd expect the December player, on average, to be better. And this is an effect we see when we compare the birthdates of junior hockey players to NHL players:

The first time that players aren't strictly grouped by birthdate is when they reach professional leagues. At this point, younger players outperform older players by a wide margin, making the jump from junior hockey to the NHL at a 50% higher rate. Gladwell mentions in his ESPN interview that "Canada is squandering the talents of hundreds of boys with late birthdays." It seems pretty clear that he's right.