In terms of competitions such as Bebras, Primary Maths challenge etc., it does seem that without any form of age adjustment, older children are clearly advantaged.
Our daughter is summer born, so at age 8 had a high merit on Bebras in the 8-10 category, and a high distinction at age 9. Clearly age has an impact..and now at 10 she has to compete against 12 year olds, which seems unfair.
Equally with the maths challenge. She scored one mark less than the 2 winter born kids who were chosen to go to bonus round, which also seems a little unfair. This must surely be a common theme where age is not differentiated in any way?