What doesn't make sense is that he was still good, off of limited training, a year and a half ago. That isn't that long. He beat Rupp in a road 5K, ran 3:35 or whatever... definitely respectable times that looked to set him up for a good 2011.
The early burnout surely would have presented itself as recently as 18 months ago, no?
And, if it is burn out... I would rather have his career before the age of 26 then a career that lasted another 6 years but resulted in never breaking an AR. You peak when you peak. I don't really think there is an early or a late peaking.
If anything, if you start training younger then what is generally done in the US, the BEST time to peak is between 18 and 24 while you still have a lot of testosterone and can recover well. Of course, to really peak then, you probably need to start training seriously at around 10-12 years old.
I don't advocate kids running a ton at 10-12... but hypothetically, with no regard to social norms, growth, etc, from a pure physical perspective, this would make sense. Get in a massive aerobic base pre-teen to late teens and this would allow you to crush a ton of intervals in your late teens while being able to recover properly because of your natural testosterone.