Kids shouldn't be playing contact football until, at the earliest, 5th grade. And I agree somewhat with the thought that the fundamentals can be learned at any time, but it's a very complex game that kids sometimes have difficulty grasping schematically. Likewise, I say let them play flag football as early as possible just to sort of begin the process of understanding what a coach is trying to tell you about assignments, alignments, etc.
I also think you're really underestimating the element of contact. A 14 year old kid going out for football his freshman year without ever having been hit, or delivered a hit, is going to be behind. He may even be a pussy, but nobody would know until he plays the game with pads. Then you'll learn pretty quickly if he's a football player or not. Usually takes a week of full contact. Seen it a hundred times.