I've come across a rule of thumb, in our culture, which says "divide his age and add 7. If she is that age or older, it won't raise eyebrows, but if she's younger than that age it will."
That doesn't mean the rule of thumb is correct in terms of the actual people involved, but it does seem to be quite a neat way of encapsulating the response of most people in our culture, regardless of the circumstances or personalities of the people in the relationship.