When a minor willfully misrepresents himself to be 18 or more years of age when taken into custody by a peace officer or probation officer, and this misrepresentation effects a material delay in investigation which prevents the filing of a petition pursuant to the provisions of this chapter, such petition or complaint shall be filed within 48 hours from the time his true age is determined, excluding nonjudicial days. If, in such cases, the petition is not filed within the time prescribed by this section, the minor shall be immediately released from custody.