Is having a co-adviser a good idea in a PhD program I'm currently doing a PhD in mathematics and have been working with my current adviser for almost a year. My adviser is at an advanced age, however, even though he still has a couple of more graduate students apart from me. My work has been going decent, but with oral qualifying examinations coming up, I'm confused if having a co-adviser may be a good idea for me. There are a couple of reasons for me to think so. 

First is the fact that my adviser is at an advanced age. Secondly, while my adviser was away for a semester last year, I worked with another faculty member, who recently told me to consider if I wanted him as a co-adviser. I've not spoken about this issue with my adviser yet. 

In this situation, I'm really confused about two issues. Firstly, will it be a good idea to have a co-adviser at all in my case, or should I continue having a single adviser while continuing my work with the other faculty member and keep him in my committee? Secondly, if I indeed choose to have a co-adviser, how should I bring it up with my principal adviser so that he doesn't mind? This has left me very confused recently and I'd love to hear some general advise in this situation.
I wonder why it is necessary to formalize an advising relationship at this time. If you are getting good advice from a second person, you are fine. If something happens to remove your principle advisor from the picture, you have a backup. 

I also wonder what you mean by "advanced age". Many grad students think that 55 or so is old. I passed that milestone 20 years ago and am still kicking mentally (though you can judge that from my writings here). 

But if you do think a formal relationship is advisable, you might tell your principle advisor (a) that you have been getting good advice from the other person and (b) s/he has suggested a formal relationship. Ask what they recommend. But don't bring up age - please. Such a formal relationship can be good for the other person, of course, and may be the principle reason why it was suggested. 

It doesn't need to be a "touchy subject".