A variety of studies show that the cutoff age for a sport and a child's relative birthdate are two of the largest factors determining future success. Kids born just after a sports cutoff date are the oldest players in their group and the most likely to succeed. In youth hockey, where the cutoff date is often January first, kids born in January are most likely to reach the highest levels (see chart below). In youth soccer, where the cutoff date is August 1, kids born in August are most likely to reach the highest levels.
Early-born children (born soon after cut-off date) are more likely to be identified as being talented at younger ages. This identification leads to more encouragement and selection for participation in special training programs or teams. Kids who are born just before the cutoff date show much higher rates of dropping out of the sport.
More information is available at: http://www.socialproblemindex.ualberta.ca/Relage.htm.