I live in Northern California as well (Sonoma County). I am a former teacher and I have many friends still in the system. They cannot think of a way to get our DS in early and I have read the CA education code several ways over and I can't find a way either.
If your DC is born Sept. through Dec., they can attend Pre-K and then skip K. You can enroll your child the day they turn 5 in to K and if that's halfway through the year and the school feels that they can go ahead to 1st, that's another way. At this time, I feel strongly that our DS would benefit from early admission. He is exceptionally tall, looks and acts like he's 2 years older. He in a mid-to-late 1st grade level already, almost entirely on his own accord, I'm sure we will be facing all of those future fears-grade skipping, pull out to higher grade reading groups, etc. Why not just allow him to start early, reducing some of the issues?
I haven't given up on the idea. I'm still working on it, like if some new hip public charter school would let him early, then he could just go to our public school in the 1st grade after early entry K. Or if a principal might consider early entry if we tested him, that could be an option.
He's already disappointed by preschool, it's heartbreaking. If there is any way I can help start his elementary years on a good foot by early entry, then I'm going try.
I didn't read other comments, but if you are wondering if your child is gifted, just think of how many families track mathematical milestones? You only do that if your mind is blown and you intuitively know it's not in the normal range of development. I never know what kind of day it will be with our DS. I have a huge box of homeschool type of materials and I just ask him which ones he wants that day, one day it's math, the next reading, etc. Every day is a new adventure. I used to spend time making projects and games for whatever his current thing was, but I couldn't keep up. I'd spend an hour making a sight words game and the next morning he wanted to do math and then the next week he somehow already knew the sight words so what's the point of creating curriculum?