2No person who is apparently under the age of eighteen years, or who is known to any person connected with the licensee's business and present on the licensed premises to be under that age, shall be admitted to or allowed to remain on those premises, so, however, that in any proceedings for a contravention of this rule in respect of a person apparently under the said age it shall be a defence to prove that at the time of the alleged contravention he had in fact attained that age.