Cal. Welf. & Inst. Code § 1735

If the court sentences a person under 21 years of age at the time of his apprehension to the payment of a fine and the fine is not paid, the court may either remit the fine in whole or in part, or commit him to confinement for a length of time permitted by the statutes relating to imprisonment for failure to pay fines. But such confinement may be only in a place approved by the Authority.