Human adulthood starts at 15 years old, and i guess the maximum age for ordinary peasants can't surpass 75 or so.
Make it so that at 15 peasants are level 1 Commoners, and each decade they live, they go up a level. Like that, maximum commoner level would be 7. But let's stop leveling at 55 - at that age, most people don't improve on their trades, so that maximum commoner level is level 5.
Make it so very extraordinary commoners level once every 5 years, and very impressive artisans level once every 10 years, but on the Expert class instead of the Commoner class.
If that's still too powerful for you, houserule that commoners have a -1 hp penalty per level (minimum 1 hp gain per level), maximum skill rank of 3 + level instead of 4 + level, can only have 1 skill maxed at a time, and take a -1 penalty to hit and AC while in combat, and have to pass a Will Save DC 10 + level/CR of the foe he's about to tackle (Double that if it's a PC class character or a non-animal monster) -1 for each peasant joining in the battle (So that mobs don't fear as much) or flee.
Assuming the previous penalties:
With the standard array, the most powerful peasant, at level 10 (and middle aged, so -1 penalty to strenght, dex & con), would have AC 10 (no armor items), strenght 10, dex 10, con 10, wis 11, int 11, cha 11, 4 to hit, and 30 hp (rolled max every level). He wouldn't have acccess to a good weapon, at the most a longsword, and even that's a stretch, because they cost too much gp (and peasant earning is measured in SP)... So assuming a lvl 1 barbarian PC with a breastplate & a wooden shield, and dex 16 & strenght 16, and thus 20 AC, the commoner would only hit the PC on a 16, or a 20% chance, whereas the PC would hit him on a 6 (10 AC - 4 attack bonus), or a 70% chance to hit. If he raged he could take him out quickly enough before the peasant even got a hit on him, although he has a chance of being downed before the peasant if he hits him often enough, or gets lucky damage rolls.
at level 2 the peasant wouldn't even have a chance anymore, or even at level 1, if his opponent played smartly and safely, using a bow, for instance. Sounds good enough?