Greetings!
Well, Jacob, there are a few questions I believe are in order for every DM in regards to the milieu's overall philosophy of what "Levels" mean.
Now, if you choose to subscribe to the always-popular, gritty, low-magic, low-powered philosophy, where essentially most everyone is below level 6 for anything, with officers, veterans, and elites being in the level 7-12 range, and only epic heroes above the level 12-15 range, then you've definitely got some intrinsic problems with addressing the issue. By the way, I'm not saying there's anything wrong with this particular philosophy.
As for myself, I long ago embraced high-level, epic play as an intrinsic and integrated aspect of my game world and milieu. This "epic" philosophy, such as it is, requires some different definitions and interpretations of "epic", "levels" and what that all means within the context of the milieu.
I have a fairly good number of high-level experts, commoners, and so on, as well as other character types throughout my campaign world. I base such considerations on the premise that once you embrace "epic" gaming, and the rules that go with it, that such considerations really do at their core make some far-reaching and pervasive demands on the entire rest of the campaign world, and every society, from top to bottom. Furthermore, I as DM don't really find most of any of the classes below level 4 or 5 to be terribly useful at simulating anything, and not terribly meaningful or useful to the societies I envision in the campaign milieu as a whole.
To my reasoning, once you walk through the philosophy, the rules, and analyze the implications, it simply demands sweeping changes. Thus, I tend to view most characters at 5th level or lower to be essentially in the "apprentice stage"--with adults of whatever class being typically in the 6-12 range, with experienced masters, elites, and such being in the 12-20 range. This then, essentially leaves the 18 and above crowd to still be unusual, but not problematic to explain or address within the context of the campaign milieu.
Having established that, then, it's not a great matter then to see how various exceptional people reach "epic" levels. However, even with this in place, there's the secondary question of why a particular someone would remain a Commoner, say, above level 6 or 8 if they indeed have the stats and raw natural talents and inclinations to work, experience and advance further.
As a side effect, in the process of broadening the definitions and such, reaching epic levels then does not necessarily mean that character X is a badass epic hero. Certainly, player characters fit the bill, but the definition changes based on the individual character's profession and background experiences. Thus, a level 25 or 30 Expert has not slain dragons, fought hordes of demons, and so on. (This also requires that you interpret experience and the gaining thereof differently)--he may have never even killed a single creature for any reason. He gained his experience through workmanship, socializing, supervising the guild, entering into politics, learning and travelng to see greater professional masters, and so on. That can all then work well together.
However, it still remains something of a professional and interpretive problem for higher level Commoners. If you view them as useless, unskilled flunkies, then it's more of a problem. I, on the other hand, see Commoners as simply common folk that have focused and specialized or rather, as opposed to experts, "generalized" to a limited extent in whatever chosen profession they have. Remember, Commoners are more generalists--that's almost the opposite definition and reason de entre of experts. Thus, a Commoner may be a blacksmith, and he may be a decent armourer or weaponsmith, but not both. Nor would he be knowledgeable about foreign techniques, or have obscure knowledge of many different kinds of work. He would be focused on a small, narrow subset of knowledge, and that's it.
Ok, having justified and explored that, then, it is useful to ask what context is such a commoner in? Is he an urban commoner, or a rural commoner? By your post, he is in the great city of Dyvers.
Ok, having said that, then, think about what single, focused profession such a commoner, whether male or female, would likely be. That then provides the basis for how this character may have been living and working through their life to this point in time.
Having worked through these ideas, it then becomes easier to craft a unique, tailored past to fill out this special character's life. Remember, though, such characters do not--despite being epic level--have to have crazy, epic and violent lives to reach their status, as long as you embrace the idea of alternative ways to gain experience. Nonetheless, though, such characters are still special, and require some detailed, well-thought out history and personality.
I hope this has helped, my friend.
Semper Fidelis,
SHARK