At young ages, a few extra months of development can make a big difference in size,strength, and athletic ability. A child who turns 5 years old in January will be nearly 20% older by the time a child born in December has their 5th birthday. In many sports, including hockey,children born in the early months of the calendar year get noticed by their coaches because of the superiority they demonstrate due to their age advantage. As a result, boys born early in the year are more likely to reach the professional ranks of the National Hockey League. In this paper Addona and Yates examine the powerful selection of effect of the “relative age” advantage.
A Closer Look at the Relative Age Effect in the NHL (PDF)
