Cal. Welf. & Inst. Code § 208.5

(a) Notwithstanding any other law, any person whose case originated in juvenile court shall remain, if the person is held in secure detention, in a county juvenile facility until the person attains 25 years of age, except as provided in subdivisions (b) and (c) of this section and Section 731. A person whose case originated in juvenile court but who was sentenced in criminal court shall not serve their sentence in a juvenile facility, but if not otherwise excluded, may remain in the juvenile facility until transferred to serve their sentence in an adult facility. This section is not intended to authorize confinement in a juvenile facility where authority would not otherwise exist.
(b) The probation department may petition the court to house a person who is 19 years of age or older in an adult facility, including a jail or other facility established for the purpose of confinement of adults.
(c) Upon receipt of a petition to house a person who is 19 years of age or older in an adult facility, the court shall hold a hearing. There shall be a rebuttable presumption that the person will be retained in a juvenile facility. At the hearing, the court shall determine whether the person will be moved to an adult facility, and make written findings of its decision based on the totality of the following criteria:
1. (1) The impact of being held in an adult facility on the physical and mental health and well-being of the person.
2. (2) The benefits of continued programming at the juvenile facility and whether required education and other services called for in any juvenile court disposition or otherwise required by law or court order can be provided in the adult facility.
3. (3) The capacity of the adult facility to separate younger and older people as needed and to provide them with safe and age-appropriate housing and program opportunities.
4. (4) The capacity of the juvenile facility to provide needed separation of older from younger people given the youth currently housed in the facility.
5. (5) Evidence demonstrating that the juvenile facility is unable to currently manage the person’s needs without posing a significant danger to staff or other youth in the facility.
(d) If a person who is 19 to 24 years of age, inclusive, is removed from a juvenile facility pursuant to this section, upon the motion of any party and a showing of changed circumstances, the court shall consider the criteria in subdivision (c) and determine whether the person should be housed at a juvenile facility.
(e) A person who is 19 years of age or older and who has been committed to a county juvenile facility or a facility of a contracted entity shall remain in the facility and shall not be subject to a petition for transfer to an adult facility. This section is not intended to authorize or extend confinement in a juvenile facility where authority would not otherwise exist.