As a random anecdote I have to consider one of my friends. He is about as unathletic as they come. He walks gangly, runs even more gangly, terrible hand eye coordination (he will sometimes miss completely objects thrown at him), probably has a max vertical of like 8 inches, and I doubt he could break 18 seconds in a 100m "sprint".
Maybe there is hidden "talent" for distance running in there, but I doubt it. He is the type of person when you see it's almost astonishing how bad athletically he is.
Yet still, he has worked his way over about a year and a half to running about 40-60 mpw. He likes running far, but doesn't really care or do much work to run fast. Just lots of long jogs. Even he, who I'd consider to be literally in the bottom few percent athletically, has managed to go under 21:30, and that's at 7000' altitude.
----------------------------------------------------------
Having stated my thoughts about some of these common examples people were bringing up I'll offer some thought on the OP.
One thing people like to talk about is the 400m speed + 8-10 seconds = mile pace per lap, with that applying to a reasonably trained runner. If you took the average person in their 20's that isn't overweight they could almost certainly run 15 seconds for a 100m, which gives them a 400m of at the very absolute least 70 seconds. So that's a mile time of somewhere between 5:12 and 5:20 which is roughly the equivalent of an 18:00 5K.
Keep in mind the OP is talking about individuals properly groomed from birth, so the actual 400m time of the hypothetical runner the OP is talking about will almost certainly be much better, especially if they are involved in general athletic activity from a young age.
Combine all this together and I think it's reasonable to say at least 18:00, maybe as low as 17:00 for the average person TRAINED FROM BIRTH.