What usually makes it much easier in Europe is that most postdoc positions are without attached teaching, so there is no pressure of that sort when you are considering leaving. Otherwise, everything seem to mainly depend on whether the postdoc position in question is an individual research grant given for your own research proposal or it is a "research associate" position attached to a bigger grant which has already started. In the first case, the starting date is often very flexible, and the leaving date can be easily changed if you have done enough to demonstrate they were paying you money for a good reason; if you want to leave MUCH earlier, say after a year with a 2-year position, then you have to convince them that it is what is crucial for your career growth - but in all cases I know the postdocs did not have problems either. In the second case (research associate), the starting date is usually supposed to be very soon (the grant is given for a fixed period, so they want postdocs to start ASAP to not fail their first year joint report), whereas with leaving early people tend to be rather understanding (but (1) it depends on the grant PI a lot, clearly; (2) my information in that respect is mostly about Ireland and the UK, that might be seriously different in Germany, for example).
Let me separate your question into two parts, and note that some good advise is already given in the comments to your questions.
First, you are concerned with the status issues and the age issues (student vs. professor, recent work vs. old classical papers, etc.) I think math is pretty egalitarian, so the status issue should not be an obstacle. The reality may be different, but often it's not an ethical issue - some senior faculty tend to be busier than grad students... Similarly, to a lesser extent, math is timeless. Unless the older work has been subsumed/trivialized by later far-reaching developments, I don't see this as an ethical obstacle either. Of course, some people retire, leave the area or mathematics altogether, die, join NSA, or stop communicating with the outside world, but that's just life.
Second, perhaps the more interesting part of your question is whether it is ethical to ask a clarification of some argument, complete proof of the lemma, etc. While others might emphatically say "yes", I think this is less clear. As with food or diet, I think this is good in moderation, but bad in large amounts, and the line is really easy to cross, and when that happens it's unpleasant for everyone involved.
To make another imperfect analogy, let's compare math papers with children. While the parents are of course completely responsible for their behavior when they are very young, as they move into adulthood this is less clear and eventually not true. While the line in this case is hard to draw (it varies in different countries and cultures), when it comes to papers there is a clear line: the publication date. In my opinion, while the paper is in the preprint form, the default position is that the author is responsible for all that's in the paper, as it undergoes public scrutiny. She/he really should answer and explain unclear/difficult points, unless there are compelling reasons not to do that (say, a followup paper with a better exposition). But after the publication date, it seems the author may answer only if he/she wishes to, and the default position is that "somebody studied the paper and agreed with eveyrthing in it". I know, life is more complicated, the authors and referees make mistakes, etc. but that's life again. My point is that you should not get upset if the author refuses to comment on a published paper.
Best Answer
If you get support (financial and moral) from both places, you should list them both.