But getting fired from a job should ABSOLUTELY be investigated, and I'd assume "talk to the boss" is the standard for that.
That's some negligence right there! (or, he left it off his application entirely, and they didn't do enough checking to verify employment at the time, and see the gap)
I also seem to recall being granted S, TS, TS/SCI, etc. is a lot easier/faster if you're in the military than if you're a contractor. Different queue, different investigators, and higher presumed loyalty to the US. (i.e. you were willing to roll the dice and potentially be stuck as a cook for 4-8 years, which a spy might not be willing to do)
I think minors aren't really investigated because they simply don't have much of a paper trail to look into. Their security profile matches their parent's more than their own. But who knows.