not surprising, very similar to Gladwell's hypothesis about star hockey players being born in the months of January - February. Students a year older than other students have a huge competitive advantage.
Especially in the early years of life, when the 11 month difference between a January baby and December baby in a class might be 1/5 to 1/10 of the child's life. That's a huge advantage, physically, intellectually and socially, for the older child.
It would be interesting to see whether the advantage of being 12 months older is higher in sport then in grades or vice versa.