Keep in mind what commoners are. Commoners are unskilled laborers; they are not blacksmiths, merchants or soldiers. They are serf, slaves and peasants. By gaining levels you become more skilled at what you do. The idea of a high level commoner is kind of absurd. I really can’t see a commoner going much above 3rd level and even that would be unusual.
The other NPC classes on the other hand I can see gaining a few levels. Most of the NPC that players are going to interact with are probably experts. These are the blacksmiths, merchants and even many of the farmers. The person who owns and runs the farm is probably an expert not a commoner. The town guard or members of the military are going to be warriors. I can see either of these classes being up to 5th level without much trouble, with the occasional 6th or 7th level character.
Most NPC’s with any kind of authority are probably aristocrats. Even in a society without nobility aristocrats will be what the ruling class ends up. Of all the classes they will probably be the ones with more higher level characters.
One of the reasons that you don’t see many high level commoners is they have a tendency to die a lot younger. Poor nutrition and harsh living conditions take their toll on commoners. Then factor in poor or nonexistent health care and your life span ends up pretty short. By the time a commoner reaches 30 they are probably in about the same shape as another class in their 60’s.