Sometimes children are bullied by children of the same age, so separating children by exact age does not help with this. It may reduce the frequency, but I can imagine other ways of solving this problem; for example having adult people near, or using cameras in schools, in case of problem looking at the evidence and actually punishing the bullies.

I only have anecdotal evidence, but it seems to me that the biggest problem with bullying is that the bullies are never seriously punished, simply because the bullies are children too, and there is always someone there to protect the child from any harm, even if that child is actively trying to ham another child. And every time the bully gets the "second chance", the victim gets a lesson in helplessness.