While I agree six PC levels is high for somebody described as lazy, cowardly and crude, some people don't seek out challenges, life forces them upon them. If he was born into an extremely harsh and violent world, he might have accumulated that much experience just by trying to survive.

As for the highest normal person, it says in the DMG somewhere (AFB right now) that most people never get past level 2 or 3, so I guess level 3 is the default ceiling for 'normal'.