1. all these random people chiming in with "i'm incredibly cool and never ran until i was 15 and then two weeks later i ran sub 18 and then a month later i broke 16 but whatever i suck" - first of all, no, you didn't. second of all, it is COMPLETELY irrelevant to Bekele.
2. there is no such thing as a vacuum. the world is not divided between the aerobically trained and the non-aerobically trained. it's a scale. yes, specificity is a thing, but an east african farmer (probably living at altitude) is going to be in WAY better shape than the average 25 year old american or european. our culture is sedentary, so people thinking of an average 25 year old's capabilities have a vastly different idea in their minds.
3. if he's a farmer in ETHIOPIA it's practically impossible, unless he has some sort of disability, that he didn't grow up running and doesn't run daily performing tasks on his farm and traveling to other farms/villages. also it is just generally impossible to say a human being has never run in his life. of course he has. even if he played no sports and even if he's the laziest farmer in the history of Africa - he's done some minimal running. we can say he hasn't trained, though.
4. and with that in mind, one of the greatest distance runners in world history, in his prime, working a physical job, probably at altitude - he'd run between 17:00-19:00 his first go for 5k.
5. now, put galen rupp in an average american office, living the average american lifestyle with a car, horrific food, saturated with alcohol, stressed, and sleeping poorly? he'd probably be at least a little overweight if not significantly obese, like the average american, and he'd barely break 30.