California Code, Code of Civil Procedure - CCP § 373

(a) If the minor is the plaintiff the appointment must be made before the summons is issued, upon the application of the minor, if the minor is 14 years of age or older, or, if under that age, upon the application of a relative or friend of the minor.

(b) If the minor is the defendant, upon the application of the minor, if the minor is 14 years of age or older, and the minor applies within 10 days after the service of the summons, or, if under that age or if the minor neglects to apply, then upon the application of a relative or friend of the minor, or of any other party to the action, or by the court on its own motion.

(c) If the person lacking legal competence to make decisions is a party to an action or proceeding, upon the application of a relative or friend of the person lacking legal competence to make decisions, or of any other party to the action or proceeding, or by the court on its own motion.