Well I wonder if Obama would try to say Biden is too old like he did with McCain. Oh that's right it wasn't really about age, it was about getting himself elected to office.
No one EVER said ANYTHING about McCain's age in the Obama campaign that I remember.
In fact, I distinctly remember footage of a rally where someone in the crowd yelled out "He's old!" about McCain and Obama stopped his speech to push back and say McCain's age was no reason to oppose him but his policies are.
On the other hand, if Rubio wins his senate race, you'll immediately hear many of the same people who warned Obama was too inexperienced suggest Rubio should run for president.
On 2016, while the Tea Party screams about Obama's "radical socialism" there's actually growing disappointment in Obama among progressives that he's too moderate and traditional a politician. People like Blanche Lincoln block a Public Option and now Obama cuts an ad for her. He throws the GOP a bone with a call for more off-shore drilling and we get an environmental disaster.
I wouldn't be surprised to see the 2016 Democratic primary be a reaction to that disappointment in some way (a mirror image of sorts of purists getting more control of the GOP). I could see Obama letdown after high expectations create fertile ground for someone like Russ Feingold or Alan Grayson, who would have the track record to credibly campaign as a genuine reformer.