Here is something to read on the subject: School starting age 'should be raised to six to prevent long-term damage to brighter children' | Mail Online

It is based on the Terman study that followed 1000 children over their lifespan. I read the original paper but can't find it again - anyways, what it came to was that kids that are more psychosocially ready for school had better longterm outcomes, less likely to drink/smoke/use drugs and less likely to have anxiety issues etc. I guess that comes down to peer pressure and kids that are that year older at high school may be less easily swayed by what other kids are doing. If you are interested I can track down the original paper.

Obviously that is just a generalisation an not an absolute. You know your own child.

From my own experience, my DS would have been in no way ready at 4.5. We attempted to start him at 5 but it was a massive disaster, but at 5.5 he was totally ready and went happily. He matured heaps in those extra few months at kindy. Like Kim's child he became a real leader - took centre stage in a production in front of hundreds of kids etc, something he just wasn't capable of 6 months earlier.