If the younger man is of legal age, capable of recognizing if someone is taking advantage of him or not, and no one is being hurt, then yes. But that's true for everyone in every relationship.

The same could be said about the relationship between an older man and a younger woman, though I think society has had more than enough time to get acquainted with the notion.

But is it "ethical" for an age gap to exist between an older woman and a younger man? Sure, why not? One of the primary reasons why the reverse age gap was so acceptable was because people did not live as long as they did now and women were married off a lot younger to maximize their child bearing years. I think this sort of practice continued into the early twentieth century and we still see and practice some of the residual effects. Also, males were a dominating sex up until the mid twentieth century. One could argue they still are, but things are beginning to change.

So now that we are at some modest point of modernity, the factors that would have made the age gap taboo are starting to haze and disappear. Women can now provide on a level as that of a male and the standard of health as increased significantly since even a hundred years ago. So, not only is it "ethical," which seems to imply some restriction against the female sex to begin with, that an older woman could have a relationship with a younger male, it seems fitting considering the level of progress achieved in society up until now.

The only thing I would like to add is why would it be not ethical, with what Aedes said as a given?

Thats because people from very different ages marring is a cultural taboo... aspects of culture are formed for a reason, but often continue existing after the situation changed Like flowers on the marriage... that tradition is because in the dark ages people couldnt have bath often so they served to aleviate the smell from so many people togheder

It is not necessarily an age difference that leads a relationship being "unethical." Rather, the moral judgment is really about whether one, or both, of the individuals are capable of discerning (or being aware of the possibilities about) the motives of the other and making "rational" choices ("with both eyes open").

Society generally abhors rape. Physical compulsion in a striking way denies the subject freedom of choice; psychological compulsion ("taking advantage of") is seen as similar.