Concerns about a very old PhD advisor?
My PhD advisor was already retired when I started my PhD. He is in his 80s now. One of my collaborators is due to retire soon; the other is in his 90s. All I can say is that everyone of them has had a tremendous impact on me. They have been the most brilliant people I know. 

So I don't see any problem at all with very old PhD advisors. In fact, my experience suggests that you should consider yourself privileged to have an old PhD advisor. A few reasons stand out, at least from my experience: 


They have a wealth of experience.
Old professors are usually kind, generous, and so eager to share with you what they know. 
They have nothing to lose. They don't have issues that younger professors face such as getting promoted, tenured, or the pressure to publish. 
Retired professors don't have as many responsibilities as they used to, and so can devote more time advising you. Of course, they probably have other interests they would like to pursue now that they are retired, but research will always be their main interest. 


All I wanted to say is that age doesn't matter when it comes to choosing an advisor. Passion is more important. Some professors manage research; some others do research. Choose the latter.