Can a 23 yr old licensed driver obtain auto insurance coverage on parent's policy in state of California?

It really just depends on the circumstances and you insurance companies underwriting guidelines.

If the 23 year old still lives at home and drives your vehicle(s), then most of your local insurance companies will have no problem with this.

If the 23 year old has a separate residence and has his/her own vehicle then you are no longer considered to have a financial interest that vehicle or their insured status. You can not insure property when you have no insurable interest.

Additionally many of the larger national standard carriers have internal rules that require your child to buy their own policy at a certain age. The company underwriting guidelines just prohibit you adding them after a certain age. This is not a legal matter but a business decision and marketing strategy on the part of the insurance company. At a certain age they want your child to get his own separate policy so the Insurance Company can begin to foster a separate relationship with them as new client.