Malcolm Gladwell started much of this with his book Outliers when he showed the impact of age/birthdates for youth hockey players and the incredibly disproportionate number of successful players who had birthdays, like your brother, that were on the cusp of a cut off, making them the oldest players in their particular age group. They were better at younger ages so they played better positions and made better travel teams and so on and so on all the Anya to the pro leagues.
This was then taken a step farther by parents holding their kids back a year for an even greater advantage.