Speaking of how aggression is good, have a look at what kind of trouble kids these days get themselves into because they've never experienced violence, assume they're immortal, and inevitably run into a big fat reminder that they are not.
Pint-sized spazz of a bully gets dropped by junior viking
YouTube keeps taking the video down, but see this fan site that has the video in the top right. Here is a permanent description.
This is just like the lack of exposure to toxins and pathogens while growing up -- your immune system is completely clueless what to do when it eventually does encounter them, goes haywire, and makes you allergic to everything.
Bone fractures are also more common among kids today, and probably not just because of their grain-only diet which robs them of calcium and vitamin D, but because of the lack of rough-and-tumble play. In such a world, their bones get the signal that they don't need to be particularly robust, so why waste the resources in making them strong? Again you can run but you can't hide from bone-fracturing situations, so eventually they'll find themselves in one and get hurt a lot worse than if they'd been toughened up years beforehand.
In a world with so little rough-and-tumble play, so little bullying, and so little violent conflict in general, young boys don't develop a mature sense of when, who, and how to fight. This little nerd probably only picked on kids just as wispy as himself. It may take longer than in a more violent world, but eventually he'll run into someone who won't take his shit, and instead of "merely" getting a bloody nose or maybe even knocked out, he is so unprepared that he gets bodyslammed and finds himself so hobbled that he can only gimp around.