To start school, you need to be 5 at the end of september (i think its the 28).
If you are a bit after that (like me), you can ask for a special permission to strart "sooner".
My parents choose not to do that(the idea is if you start a bit after you are more mature. And I can tell you, when in high school, that difference "small" difference in maturity really shows up!). That is why I was the oldest in my class.