I've looked at the age-of-consent and other sexual assault laws of several states since I started law school three years ago. It seems to me that these are very difficult lines to draw, and states have come up with different ways of stating the rules and different ways of handling some of the hard questions.
Some states do account for proximity in age in prohibiting what is commonly called "statutory rape." Take Colorado, for example, where different rules amplify or remove punishments for consensual sexual conduct between people of certain ages. For example, it appears on a quick reading of the statutes that in Colorado, statutory rape has not happened if there is consensual sexual activity in which the 'victim' is 15-17 years old and the age difference is less than 10 years, or if the victim is less than 15 years old and the difference is less than 4 years (though in my mind the latter provision opens up what I think are some kinda creepy possibilities -- but other laws directed at child abuse may still apply to those situations, I don't know).
The relevant statutes are all linked off of
this page. See Colo. Revised Statutes 18-3-402, 18-3-404, 18-3-405. They may not be up to date. I don't know how other statutes might apply to make the same sort of activity possibly illegal legal, but they might and in some cases surely do.