The parent of an adult child with disabilities can file in Superior Court for a conservatorship. This allows the parent to make legal decisions for their child even after age 18. You will find forms and procedural information on the San Diego Superior Court website, under the Probate section.
Practice Materials: CEB OnLaw
CEB OnLaw Pro is a rich source of California practice materials. For limited conservatorship practice materials, navigate to Trusts & Estates > California Conservatorship Practice > Chapter 22: Limited Conservatorships.