As the other person commented, it’s the opposite. At young ages, the difference of 11 months is a huge percentage.

Let’s say we have 2 players we have to choose between. Player A and Player B. Both were born in the same calendar year, so for many sports they are grouped together for divisions and such.

Player A was born in January

Player B in December.

Let’s also imagine that they are 8 years old, trying out for their first travel team. At this point in their lives, 11 months of extra growth has given Player A a slight advantage, and the coach picks him for the team. Player B doesn’t make it, but sticks with the sport in recreational leagues.

Player A however gets more specialized coaching, more practice since they’re also playing rec league and travel, and they get more experience playing at a more “serious” level.

Well now it’s a year later, and time for tryouts again.

Player A still has 11 months of growth over Player B, but he’s also got a year of more intensive, in depth training, and more time playing the sport. Even though Player B has been practicing his best, and playing rec level, he just hasn’t developed at the same rate. Plus, he’s still 11 months behind on his physical growth. This gap in skill, experience, and maturity has prevented him from making the team again.

A year later, their 3rd tryouts, will show the divide between the two players has grown even further than the previous years, and any hope of Player B catching up is looking unlikely without some serious, specialized training.

This cascading effect continues throughout their lives, and the effects can be seen greatly when puberty enters the equation, and the rapid development of some players, but not others, coincides with important milestones, like making a high school sports team, or some other big event.