If you follow Advanced Fighting Fantasy, you will probably know that they released the Combat Companion.
As well as having plenty of great stuff for combat, it also had rules on creating characters with powers above or below the standard heroes in the core rulebook.
The lowest "level" is civilian level - basically a 0 level commoner.
I love low level games. The lower, the better, so I have created a civilian level adventure for Advanced Fighting Fantasy. I want to create enough adventures so the heroes get enough xp to go through to veteran level, which is what the heroes in the AFF core book start as.