Hi--It is such a big decision and always takes a lot of consideration. As a teacher I can tell you I have never met a parent who was sorry they waited and I've met many who were sorry they sent the child 'on time'. California is way behind with respect to cut off dates! As another poster mentioned, Kindergarten these days is what 1st grade used to be. There is a lot of pressure to achieve certain benchmarks in order to go on to 1st grade. When children start at age 4 it is extremely difficult for them to reach these goals and many have to be retained for a second year in Kindergarten.
I have four children, with birthdays in January, May, August and October. Where we lived in NJ, the cutoff date was Oct 1. So my January girl and Oct 11 boy waited the year and were always very successful and confident and a great students. My second boy had a May birthday, and I was reluctant to send him even so. I was pressured into it and he paid the price. He did OK in K, but was moved from 1st back to K midway through the next year. He settled down somewhat after that, but was never a good student and never really liked school. My last was an August girl and she started right when she turned 5. I think she would have done better to wait because as the youngest, she is the most immature of my kids.
Sorry to bend your ear with so much of my story! Bottom line is, if you have ANY doubts, wait. There is no real down side to this. It can't hurt and may well help. Good luck with whatever you decide!