you just can't compare a 25 year old farmer who only walks around the farm for the purpose of chores to a high school kid. High school kids are active in gym a couple times a week, play games like basketball with friends, ride bikes, perhaps play soccer for several months, etc. Also, as a teen, the muscles are very elastic. I can remember as a teen I could always fire a baseball near top speed on my very first throw after 5 months off from it. In my 20s, it turned into needing a 20 minute very gradual warmup just to get going.
This is an almost unanswerable question. 20 minutes is a reasonable guess, assuming Bekele did something like soccer as a kid, but has done zero athletic activity involving running for the past 6 or 7 years. But really, who knows. This is not a likely scenario.
My own personal experience with this is that at 28 years, after 11 years away from any sports or gym classes, I ran 2.6 miles at 9:06 pace. I am a nerd, so I have recorded every run since that day I started 10 years ago. I am zero talent. 10 years later, I have a 19:22 5k pr. Though I only started doing track workouts last spring.