Is having a co-adviser a good idea in a PhD program
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".