The only exception for something like this would be if someone who is 16 is dating someone who is 18. The reason being is that it's mostly legal in the United States as long there is nothing sexual.
There is this age rule some people go by. Take half your age and add 7. That is the youngest you should date according to some people. Is right? Well, if we're talking about that area when you turn 18, yes. If you AND your partner are well over 18, then it's fine, but if say person A is 20 and person B is like 50, it's completely legal, but socially... It's weird.