Mid-30s is when it really hits you. Look at how much speed Lagat has lost in the last few years--he can still run well in long races, but he doesn't have the speed anymore.
Geb was like that in 2008--right there in the Olympic 10k, but just couldn't kick with the young guys. Now age is starting to eat away at his body too. Can't stay healthy...he probably COULD still run 2:06 or 2:08, but he is tired of making excuses and doesn't want to keep disappointing his supporters. Tragic, really. On one hand, one of the reasons he was on top for so long was because he was beloved by so many millions around the world, and now despite being physically able to continue running and competing at a very high level, does not want to run if he cannot be the very best. Not for his own good, but to please his fans.
I think Gebrselassie, along with El Guerrouj, Bekele, and Paul Tergat, will be remembered as the man who brought running into the 21st century. Smashing records and setting totally new standards for what entails greatness.