Farah looked to be at least 3-5 pounds too heavy. Can't even see his cheekbones. No wonder he wasn't competitive. 4% too heavy = 2.5-3% slower. It also means he'd blow up going out too fast trying to hang on. Simple.
As for why Africans often peak earlier? Their bone mass peaks earlier. Whether it's a genetic difference or due to their environment, I don't know. Personal bests at the elite level almost always coincide with peak bone mass. This is true of nearly all sports that are not extremely tactical.