There isn't any rule - this half your age plus seven stuff is rubbish.  The acceptable gap is whatever you feel comfortable with.  I know a 40 year old guy who goes out with a 25 year old.  They don't look like an odd couple at all and they break this so called rule.
	
		
			
		
		
	
				
			 
	 
  
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		

 
 
		
 
 
		