It depends on particular people in question.
I believe.. not always it would be wrong, and not always it would be right. And I think age difference would not necessarily be the thing that is the problem in case of it when it is wrong... but problems related to it. Is the younger person too naive to make well decisions about such a relationship, for instance? Not all 17-year-old people are naive, but indeed some could be easy taken advantage of. That is only an example, but there are many such things to consider about it.