Children must be 5 by Oct. 15th in my state (I'm not sure if that is true everywhere.) As a former teacher, I can tell you that the more time they have to learn and develop before they come to school the better off they are when they get to school. Just learning to abide by a school routine, social interaction, and the ability to focus on various tasks, is very important. The more mature the child is when he/she starts the better. Experience shows that boys tend to mature more slowly than girls do.
Don't rush it! It is not free babysitting..........