So now its 1-8 years old. Why not 9 or 10? Sure, raise it to 18, then 21 and then 40. Pretty soon you'll realize you can't make generalities like that and state them as truth. I also notice you're using white Americans for your examples (at least it seems that way to me). Because in many other cultures throughout time (wait, your theory doesn't include dead people) males by age 8 have/had many "adult" responsibilities.

What about a squire? Or a drummer boy? Did they not face battle at a young age along with full grown adults? Didn't women get married at an early age? Didn't boys become kings of great power?

By limiting yourself to modern white America, your theory is flawed.