I have always been bothered about this, but right now I feel it's a better time in my life to speak of this annoyance. In the US you can take part of the lottery at age 18. You Can Drink alcohol related beverages at 21. BUT you can't enter a Casinos Main floor until the age of 21. (In certain cases 18, but very few casinos do this in the U.S)
~P.S.~ The only reason I'm writing this thread was that I was on the main floor and my parents left me for a minute (Literally A MINUTE! I check my time) and the security girl comes up to me Miss you must go downstairs. I don't see your parents visible, so you must go. (Let me tell you this whole casino smells like smoke and the bottom floor smells like it but 10 times worse.) I wished casinos "family floor" had something to do other than I get pushed off my seat by a man in his early 20's (which I was seated on). This "family floor" Basically had 6 bowling lanes, and an arcade room where each play would cost an equivalent of one to two dollars each play.
