It's Setting-dependent

In most settings, level isn't synonymous with political power. Kings can be level 1 and farmers can be level 20. It's often easier on the community (and on the setting's verisimilitude) if the most powerful folks are the folks in charge, but that's never mandatory. A highborn, well connected level 2 wizard could be in charge of the wizards' guild, and a down-on-his-luck level 20 fighter (perhaps the victim of amnesia magic or an effect that removed from him his gear) might be earning his 2 sp per day working as bouncer at the Rusty Gorgon.

Two Examples

• The Eberron setting for Dungeons and Dragons 3.5, Pathfinder's immediate antecedent, had very few high-level characters, owing to disastrous conflicts in the setting's recent past. I'm no Eberron expert, but I think most published characters for the setting were levels 10 and below.
• The Forgotten Realms setting, originally published for Advanced Dungeons and Dragons and updated for every edition since, came to be inhabited by so many high-level characters DMs sometimes struggled to find things for their low-level players to do at all because, it seemed, a high-level character should've handled it already--I mean, there's one right over there, after all.

One of my disappointments with Pathfinder is its lack of demographic guidance. I, too, am curious, for example, just how many summoners the designers think should live in a village. While I think it's brave of the designers to leave this to the DM instead of mandating it, at least some guidelines like those available for Dungeons and Dragons 3.5 in the Dungeon Master's Guide (137-41) (but not made available as part of the d20 SRD) would've been useful, if only for comparison.

Suggestion: Reverse-engineer

If writing a campaign instead of setting, gear the setting to the campaign, not vice versa. Figure out what you want the characters doing in the campaign at levels 1-5, 6-10, 11-15, and 16-20, and make the NPCs of levels appropriate to when they're going to be more important to the campaign. This way the NPCs can be encountered earlier and be untouchable yet the NPCs can be of the appropriate levels when the PCs reach that point in the campaign.

So the challenge with this question is that it's not just about classes but also about resources expended. You could be a Fighter choosing to use leather armor and two daggers or you could be a shield and heavy armor Fighter with the same stats and have a very different AC.

The other complication here is Touch AC vs. Flat-footed AC. Most characters are good at one or the other but not both.

That stated, there are really only a couple of builds that really matter here for benchmarks, so let's run some comparisons. Note that it's really less about "class" and more about "feats" + money.

The Tank: Heavy Armor + Shield

I modeled this one as a Fighter spending ~75% of their cash on defense.

• Level 3: 24 (Full Plate +1, Heavy Shield, Dex +1, Dodge +1)
• Level 5: 28 (Heavy Shield +1, Ring of Protection +1, Amulet of Natural Armor +1)
• Level 7: 31 (Full Plate +2, Heavy Shield +2, RoP +2, Dex +2)
• Level 9: 36 (Full Plate +3, Heavy Shield +3, AoNA +2, Dex +3, Dusty Rose Ioun Stone +1)
• Level 11: 40 (Dex Belt => Dex +4, Armor/Shield +4, Ring/Amulet +3)

The Light Armor Dex build

As above this Rogue is spending most of their money boosting their AC. In this case, they benefit greatly from the Dex Belt as well.

• Level 3: 20 (Mithril Chain Shirt +1, Dex +4, Dodge +1)
• Level 5: 23 (Dex +5, AoNA +1, RoP+1)
• Level 7: 25 (Chain Shirt +2, AoNA +2)
• Level 9: 28 (Chain Shirt +3, Dex +6, RoP +2)

The Wizard/Sorcerer (with some Dex)

As above, this person is spending lots of their money on basic defensive gear and using the Dex to benefit their ranged touch attacks.

• Level 3: 15 (Dex +3, Bracers of Armor +1, RoP +1)
• Level 5: 18 (Dex +3, Bracers +2, AoNA +1, Draconic Natural Armor +1)
• Level 7: 21 (Dex +4, Bracers +3, AoNA +2)
• Level 9: 25 (Dex +4, Bracers +4, RoP +2, Draconic Natural Armor +2, Dusty Rose Ioun Stone +1)

Some Takeaways

These are "max" numbers not median numbers by any stretch. In fact the numbers should reveal several things:

• Dex is really important for AC outside of Heavy Armor characters, this constitutes most of your AC.
• By level 9 the spread on AC is huge. This actually causes big problems If your fighter has AC 30 and your Sorcerer has AC 18 (typical) then you end up with creatures that can never miss the Sorcerer if they can also hit the Fighter.
• This problem only gets worse as levels go on. At some point, you stop attacking fighters and just start casting spells at them because your odds of success are better.
• Likewise creatures become impossible to "balance". Take a CR20 Red Dragon, Wyrm, it has a bite of +38. Your Sorcerer is never going to have an AC to counter that. It basically means that the Sorcerer cannot get into melee combat or it will be ripped to shreds. It doesn't matter if the Sorcerer has dropped 500k on AC items or 50k on AC items.
• Some math (Bracers +8 => 64k, AoNA +5 => 50k, RoP +5 => 50k, Belt of Dex +6 => 36k). That's 200k in AC pump for a net AC of 31. Add a Shield spell and you're still under the threshold. You'll need Morale or Luck bonuses from a Bard or Cleric bring that up to 40+ and even then, you'll probably get hit.

There are 4 defenses

• AC is not the only defense. In fact, as levels go up your Ref/Fort/Will saves become far more important. No amount of AC will save you from a Wail of the Banshee.
• Experienced players understand this problem. At some point it becomes pointless to try to max out AC and instead you throw the resources into other things. Cloak of Resistance is far more important than Ring of Protection. At some point, instead of another +1 to your armor, you buy Energy Resistance or something to give you Damage Resistance.
• There are other ways to avoid getting hit. Mirror Image, Invisibility and Fly are all important abilities that allow weaker AC creatures to survive combat.
• AC is really only one part of the puzzle. For many combats, keeping certain characters protected is just as important.