Back in the old D&D days Adventurers stood out in the crowd. "Normal" people, content to live their lives without ever worrying about how many torches, 50 foot sections of rope, and iron rations they should include in their equipment, were all happily 0-Lvl. Famous pianists, aging farmers, and mercenaries all fit there.
Of course, things were different in skill-systems. In Traveller, for instance, skills were always a function of age (and number of terms of service). In Rolemaster the common city guard was a 3rd level Fighter -- strange that they'd hire that band of 1st level "kids" to go explore the ruined tower.
With the advent of skill systems in D&D we end up with "NPC Classes." That famous pianist? A 5th lvl Expert. The farmer? A 9th level Commoner. The mercenary? A 3rd level warrior. That pianist has two attacks per round!
I'm not opposed to NPC classes. Far from it. The Adept class is great for making non-adventuring priests or wizards, the Aristocrat class makes socially able nobles who still are no challenge for the PCs in power. And unlike PC classes, the NPC classes get no bells and whistles as they increase levels. In a game like Pathfinder, where almost ever PC level has a tangible benefit, there is a world of difference between an 8th lvl Fighter (PC) and an 8th lvl Warrior (NPC) even if they had matching stats and gear.
The Adventurer, Conqueror, King System uses Proficiencies as a mixture of skill and feat. So there are Proficiencies like "Combat Reflexes" which give a mechanical benefit, however there are also "skill-based" profs like Perform, Labor, or Craft. When taken multiple times these profs increase the chance of success.
0 Lvl characters get four (plus additional profs for high Int) , but they can't stack just yet. The older the NPC is, the more profs they get. Thus our aging farmer may have Proficiencies like this:
Animal Husbandry x2
Although bent and grey from decades of hard work, this NPC's Proficiencies reflect a highly skilled farmer. The requirement for a 9th lvl Farmer with two-dozen hit points and multiple attacks is neatly avoided.