It looks to be a valid folk formula It should go back in Only 98 Google hits ten for version with numerals for that exact phrase but they seem to show it as a widespread if not A list meme I can see why it works pretty well it approves a 14 year old dating a 14 year old 16 15 18 16 20 17 22 18 26 20 30 22 40 27 etc that seems to match about the usual limit before people or the minor s mom will give you THAT LOOK In other words its a pretty good objective description of the limit of what s considered proper which I would think would be pretty darn useful to a reader Obviously we are talking about the United States and or the West in general if necessary that could be stated in the English Wikipedia there are many articles which assume that you are talking about the West unless otherwise stated