Lukesmum, I would never have believed someone who told me I would wrestle with the decision. My DD is a May birthday and a very bright, social little girl. She was even accepted immediately into two very selective private schools. But she will start school the year she turns 6. For all the reasons 3littlemonkeys wrote (great post Kell ).
We consulted with her preschool teachers extensively.
12months on I'm glad I gave her another year of play. Her little persona is so much more developed. Having seen the change in her I'm really happy with my decision. But hindsight is a great tool...in hindsight. I did worry about her being bored, and she (and I) is/are definitely ready for her to go to school.

Totally agree with Trill about nothing different in their learning as they start school. I was the youngest myself in my year and excelled at the actual school bit. However, I've worked with teens. That was my decider. There's a whole lot of difference in kids when it comes to the teens.