Can an 18 year old get a copy of his a birth certificate in California?

Yes. In the State of California, authorized copies of vital records can only be obtained by the individual named on the record, parents of the individual named on the record, and certain other individuals or entities specified in law. All other requestors can only obtain informational copies. All copies issued are "certified copies."

Since the age of majority in California is 18, you can obtain a copy yourself. You can either mail in an application to the California Department of Public Health, Vital Records Division or submit your request to the County Recorder's office in the county where you were born.