Yes, there's such a thing as a normal 20th level Wizard. It's the halfway in Wizard progression, just like how 10th level is halfway for the expected lifespan of a Bard and 5th level is halfway for a Fighter.A normal level 20 character? Is there any? Of any class?We could come up with convoluted reasons if we want. Could be fun.
Moe the farmer is a 20th level commoner. See, three thousand years ago, Moe accidentally pissed off a passing fey lord, as tends to happen in these kinds of stories. Moe was then cursed with immortality, but never to rise above his current means or station. Moe's learned a lot about farming in the millennia since his curse, but he never can seem to get the knack of anything else.
*Tosses popcorn into mouth*
Okay, that's a situation I could accept as creating a high-level commoner. XD It doesn't do anything to explain how a normal Commoner might rise to higher levels, but at least for one of them, it works.
I think we might be playing the game differently, and that's fine. But, in case you (or anyone else) didn't understand me, I'll try to explain.
In my games, level 20 characters aren't normal.
First off, any PC that would reach very high levels wouldn’t be normal. They're PCs! They are never normal. They are rather exceptional persons, willing to set out on life-threatening adventures at level 1. After countless of adventures, they're bound to have experienced quite a lot by level 20. Dealt with powerful fey, slayed demons, toppled kingdoms, travelled the cosmos, helped thwart ancient prophesies. They're not run-of-the-mill people.
Same goes for NPCs. In my current campaign, there's a metropolis. Although there might be a few more, I've only considered one NPC in the city a 20th level character. She's an oracle and the latest in a 2000 year old line of custodians of a near-divine bridge. That ain't normal. That's unique.
In my world, no NPC is going to reach level 20 by being unassuming. Some are going to be (ex)adventurers. Others might be a bit more domestic, but still experience quite a lot. No wizard is going to stay a simple master of the wizard's guild, silently toiling with his spells and selling simple magic advice until level 20. I mean, there's at least going to be an experiment that goes wrong, unleashing foul fiends as result sometime during that career. A level 20 fighter isn't some simple watchman who happened to get really good at violence, she's going to be quite a legend.
That goes for commoners too. Sure, level 20 commoners ain't common. They're very rare, most likely rarer than high-level NPCs with PC classes. But they might exist, and they will have had a life of a strange mixture of the ordinary and the bizarre.
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On another note, I don't get the idea that high-level (or even higher-than-fist-level) commoners doesn’t exist because the pick up other classes, such as expert. Sure, some will do that. But experts get a lot of skill points. Most skills are adventure related, and those who aren't are social. A farmer ain't going to need Disable Device or Spellcraft, and he might not even need Diplomacy or Appraise since the important interactions he does are all covered by Profession. But he will benefit, however slightly, from a higher Profession rank (and a few other skills, such as Handle Animal). In my world, and I pretty much guarantee the real world too, the know-how a farmer can pick up through experience and then put to use is astonishing. There's a lot to it. And a level in Expert raises his Profession no more than one in Commoner.