Published paper containing well-known results I recently found a paper published in an high-impact journal containing as a main contribution a result which is rather well-known among the people working in the field. (In fact, this result has been firstly established in a paper in the 60s).

On the one hand, I think that the authors of the paper did not do that on purpose. In fact, they are not expert in that particular research field, and so, they probably were not aware that this result was already established when they wrote the paper. 
On the other hand, being myself a researcher working in that field, I would like to 
point this out to the authors and/or to the editorial board of the journal, so that the readers can be referred to the original paper establishing this result.

At this point, I was wondering what the right thing to do is. I would really appreciate any comment/suggestion.
It is simple enough and not rude to just send them a note with a reference to the older work. "In your recent paper, you haven't discussed the relationship to ...". 

And note that even if they have some fault, which you don't suggest, it is shared by editors and reviewers who, themselves, missed the older work.