Don't know how it is in the US or Uk, but in Aus we have a fairly straight forward law that deals with the 17 yo 18 yo problem.

If the child is sixteen or seventeen the age difference is allowed to be a maximum of four years, under sixteen it is two. The age of legal consent is still 16 but if a 17 yo and a 15 yo get at it, the 17 yo can't be charged with statutory rape, but an 18 yo can.

I think I just made a straight forward law bloody confusing!