Can a 14-year-old decide physical custody in California?

There may be some variation in different jurisdictions but generally, a child cannot make that decision. The child can express a desire to live with a particular parent but it will be up to the judge to decide if the other parent, presumably the one with physical custody, doesn't consent. A judge is under no obligation to approve of the child's choice. The judge will decide what placement is in thebest interest of the child.