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.