I'm guessing it's to do with certain frequencies; you see when we are born our frequency hearing range is from 20hz to 20,000hz. This gap gets reduced more and more as we grow older (as if our hearing is getting worn in).

So I'm guessing that at the age of 20, there are certain frequencies you can't hear as well as someone who's five years younger. Although I still don't see how this works properly, as there'd be a lot of exceptions...

Edit: BTW, I don't really see what you gain from having someone over the age of 20, not hearing your ring tone. If you were so desperate you could just put it on silent...