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.