The 2012 Olympics have been a disappointment for Michael Phelps in one sense—he's yet to earn an individual gold medal—and a triumph in another, inasmuch as he's set the Olympics' all-time medal record while in London. Which is more than most people have done in London, I think. He's got more events to come this year, but we're getting round to a question that will dog him for the next four years: Will he compete in Rio in 2016? He says no. Mark Spitz, among other people, says yes.
I can understand wanting to avoid the rigors of Olympic training for another four years, but no matter what happens for the rest of his London experience I can't imagine him leaving it and thinking: Okay, I'm satisfied. Spitz knows from which he speaks—after retiring at 22 years old, he came back 19 years later to try out for the 1992 Barcelona Olympics, which he just missed.
Wanting to retire is one thing. But knowingly giving up your last chance to compete at a world-class level, in the endeavor that is going to define the rest of your life no matter what—well, 31 just isn't old enough for me to see him turning away from it.