I can only echo what Didi wrote. Rafa is 27, which is the age many players take a step back. But his "real" age is more like 3+ years older due to wear and tear. I like the idea of one last rise to dominance, perhaps this year and through the clay season next year, and then falling in the second half of next year and struggling along for another year or two and retiring at age 31-32. I could see him missing Roland Garros due to injury one year and calling it quits shortly thereafter.
As for Roger, I also agree that we need to see how he does in 2014. If he can adjust to either being a #6-10 player or return to #4-5, then I can see him sticking around until the 2016 Olympics, even beyond if he finds a groove. But as much as he loves the game I don't see him sticking around if and when he slips out of the top 15 or so, and when he starts losing frequently in the first week of Slams. But he's such a champion that I really wouldn't be suprised to see him stabilize as a top 10 player for another 3-5 years, with maybe even one more Slam win in him. I've always thought that 18 seemed like the number he'd retire with; 2014 seems like the year he'd have to do it.