Section 15.20 - Effect of ignorance or mistake upon liability1. A person is not relieved of criminal liability for conduct because he engages in such conduct under a mistaken belief of fact, unless: (a) Such factual mistake negatives the culpable mental state required for the commission of an offense; or(b) The statute defining the offense or a statute related thereto expressly provides that such factual mistake constitutes a defense or exemption; or(c) Such factual mistake is of a kind that supports a defense of justification as defined in article thirty-five of this chapter.2. A person is not relieved of criminal liability for conduct because he engages in such conduct under a mistaken belief that it does not, as a matter of law, constitute an offense, unless such mistaken belief is founded upon an official statement of the law contained in (a) a statute or other enactment, or (b) an administrative order or grant of permission, or (c) a judicial decision of a state or federal court, or (d) an interpretation of the statute or law relating to the offense, officially made or issued by a public servant, agency or body legally charged or empowered with the responsibility or privilege of administering, enforcing or interpreting such statute or law.3. Notwithstanding the use of the term "knowingly" in any provision of this chapter defining an offense in which the age of a child is an element thereof, knowledge by the defendant of the age of such child is not an element of any such offense and it is not, unless expressly so provided, a defense to a prosecution therefor that the defendant did not know the age of the child or believed such age to be the same as or greater than that specified in the statute.4. Notwithstanding the use of the term "knowingly" in any provision of this chapter defining an offense in which the aggregate weight of a controlled substance or marihuana is an element, knowledge by the defendant of the aggregate weight of such controlled substance or marihuana is not an element of any such offense and it is not, unless expressly so provided, a defense to a prosecution therefor that the defendant did not know the aggregate weight of the controlled substance or marihuana.