My grandson started Kindergarten when he was 4. He did not turn 5 until December 31 which is really late. We started him in private school because the public schools would have made him wait until the following Fall. He was academically ready for Kindergarten however. This year he started 2nd grade in a public school and despite the fact he is the smallest and the youngest (he just turned 7 but most of the kids are already 8) he is still academically on target. He had a few issues with adjusting to a new school and new classmates but overall he is doing well now. If we had it to do all over again we would start him early rather than opting to wait until he was already five. Of course, with public schools you do not have the option of starting early. In the past the cut off date was December 2 for turning 5, but I think it will be changing in 2011 or 2012 to September 1. I have attached a link to the current laws for California below (from the Education Code) and also to an an article about the upcoming change.
I really think that it totally depends on the child because being academically ready is not always enough, but on the other hand waiting can create problems as well.