If the only criteria is qualification to be President -- ie, experience and proven skill in matters foreign and domestic -- he will choose Richard Lugar or John Warner, among sitting Senators. Or he'd go off the map and choose Warren Rudman, John Danforth or Bill Cohen. Perhaps Colin Powell. I know. Most of them are "too old", especially on a ticket with McCain. Aside from Lugar, most are too centrist to be on a ticket with McCain.
Yes. I hope very much that he chooses another old man as his running mate. A retired old man would be even better.