1st of all, every country has its own regulations, which are designed for a reason. 

2nd, your child might be smarter than his age. However, being in a class with children, who are older than him/her، have stressful social consequences for him/her on the long run. When the rest of his/her class are teenagers, it will be very difficult and stressful for him/her to deal with teenagees while he/she is still a kid!