Survival of the fittest man. They've been sending their kids alone into the wilderness for centuries (if not millennia). Only the strong survive to breed in that death world where aggressive, superpowered animals will attack you the moment you step in the tall grass, so even the kids in that world are just plain tougher and more resourceful than most real life people. Even a relative chump like Ash is still in remarkably good physical shape and is an expert outdoorsman; he just sucks at pokemon battling.