Any experience in working with aged emeritus supervisors?
Quick Answer: The downside might be his age, but the upside is plenty!
It is obvious that the age might be a factor, however you are not dealing with a dance instructor or a body builder here. Look at Prof. Stephen Hawking, beside his age, there are other limitations; but everyone listens to him and want to learn from him. Put his name on a conference, and people will line up. The following points popped up in my head:
Experience: Being dominant in a field for any amount of period, let alone 3 decades, is not easy. What you will get is a world of experience and vision (my second point here).
Vision: It is not all about hard work to be on the top, the individual needs to have a vision of the field as well. You will have the privilege to lean how he come up and deal with research issues. Believe me, I had supervisors that did not have the 'vision' part, and working with them was not pleasant.
Less Politics: He already 'done it all', and therefore not looking forward that much to the next promotion or something of that sort; what you get is a person that everyone knows that he is the leader in a field; so all the help you get you from him; you get it through no or much less jealousy and/or office politics.
Decision On Your Overall Career: If you get a position to work with a young supervisor, you can't truly answer the big question: What I will do after my PhD?. However if you work with an experience academic, you can see yourself after 20/30/40 years. You can see how hard you need to work to get there and is it something you want to do or not.
Conclusion: Take the position, there might be a little downside because of his age, but the positive points are plenty.
It's going to depend on the person, of course.
Is he still actively engaged in research? How much time does he spend on campus?
Does the university have a policy that professors have to retire at a certain age?
Go and talk to him--ask him about his mentoring style and how often he'll be available, etc.
I sometimes think gathering information on your future supervisor creates unnecessary bias. There is nothing like first impression, if you already have some mis-conceptions about this person, it might ruin any chance of great collaboration you might have had with him/her. I believe checking his publications show you if he/she collaborated with other researchers in the past or not (does he/she have a high number of single author publications or a number of the publications are collaborations? with their name appearing both at first, middle or last of the author list in different papers?). I believe working with such a renowned person in your field of study is a special opportunity you can benefit from professionally and should just put your best collaborative behavior and take a chance. At the end of the day every personal information you get from past co-workers is not without bias.