What's wrong with McCain? Yeah, he'll be even older. But if he picked a highly experienced very qualified vice presidential candidate and kept detailed information it wouldn't be such a bad deal. You get McCain's experience at the risk of his health. When his health goes down you have a fall guy with almost as much experience who's got all the information of what's going on.
I just don't know if McCain wants to run again, especially in 2012. I see it as an easy win for Obama if he manages to do anything at all, like Bush in 2004. (I also think that's why you never saw Hillary go in on the 2004 election, she was betting she couldn't beat Bush.) And if you put McCain in 2016, he's upto what, 90? You might be hitting a record there.
But I could see him running in 2012 as "the fall guy" if it's obvious the republicans cannot oust Obama. Palin would also be good in 2012, it would keep her in the media and she's certainly not too young for a 2016 run. Maybe she'll be read one of those "How to be a politician for dummies" books first.