that he believed that the person was not under eighteen; and either that he had taken all reasonable steps to establish the person’s age or that nobody could reasonably have suspected from his appearance that the person was under eighteen. For the purposes of subsection (2) of this section a person shall be treated as having taken all reasonable steps to establish another person’s age if he asks the other person for evidence of his age unless it is shown that the evidence was such that no reasonable person would have been convinced by it.
The amendment made by this section does not apply to any sale of intoxicating liquor made before the coming into force of this section.