All licensees operating a Class A, Class B or Class C restaurant, buffet, lunchroom, lunch counter, public dining hall or place where victuals and food therefor are prepared or manufactured for consumption on the premises shall permanently exhibit in clear view of all patrons and customers a printed sign which shall read as follows:
"City Ordinance. Alcoholic drinks will not be sold or consumed by persons under the age of twenty-one years."
The letters of the sign shall be not less than one inch in height.