I'd like to counter this with a theory I have:
I think people overestimate age based on looks. With respect to Kipchoge, he is Kenyan. What's a Kenyan dietary staple? Ugali. What is ugali? A carbohydrate-heavy food. The higher percentage of carbs in your diet, the more likely you are to have sugar in the blood. The more sugar in the blood, the more likely it is to glycate. The more glycation, the more likely it links proteins in the skin. The more glycated proteins in the skin, the more wrinkles. Insulin response is improved with exercise, but only to a point, and nobody beats time.
Couple with this the fact that all these guys have been running at a high level for many years. That's way more oxygen being burned than the average 30mpw hobby-jogger, or somebody whose sole exercise is walking. More oxygen being burned means a greater chance of free oxygen radicals floating around. The more free oxygen radicals, the more likely some of them are to be able to bond with water and form hydrogen peroxide. With increased levels of H2O2, the body will compensate, such as increasing production of anti-oxidizing enzymes like catalase...but only to a point. So where's it all to be dumped? I posit that the answer is the hair folicle, since it has been found recently that gray/white hair is from the folicle literally having the color bleached out of it with accumulated hydrogen peroxide. This may account for appearances of pre-mature aging in runners who have been training at a very high level for many years...such as Eliud, Kenenisa, and even Ritz these days.
Too make matters worse, there are actually very few foods that deliver vitamin E in appreciable amounts. Example: You'd need to eat about 1.5 pounds of spinach leaves to get the "recommended" daily dose of vitamin E necessary just to keep the "average" person's red blood cells from oxidizing while at rest. Sure, there are vitamins, supplements, and fortified foods. But how common are those in East Africa compared to America?
I bet that most East Africans are the age they say they are, but just lead a very different lifestyle from most Americans or other westerners.