Hence why I put the poll in this thread: What is your default image of a level 1 Fighter? The first thing that comes to mind?
Well, let me turn that around for a second. What do the game mechanics tell us the 1st level fighter is? Compare that with a commoner, and what do you see?
The game mechanics... suggest that the 1st level fighter is notably tougher than a typical commoner, but not outrageously so - a commoner has 4 hp, a st level fighter has at least 6, more probably 8. The fighter has a second wind, the commoner doesn't. The commoner has a melee attack with a +2 on the die, the fighter has at least +2, probably more like +4, and some other weapons style trick up their sleeve. The fighter has some equipment the commoner doesn't.
So, you know, the 1st level can be a farmhand... but not just any farmhand. He's the farmhand who, when all the farmhands get together in the hall at the end of the day for a near-beer... nobody messes with him, because he can kick the butt of any other single one of them - he only loses the fight if they gang up on him, and he'll take someone else down before he goes. If a tussle breaks out, maybe the village blacksmith can take him down, but maybe not. Maybe one of his parents was a soldier back when, and taught him a few things. Maybe he's just got a bit more potential. Maybe he's been lightly touched by fate, or something.
Or, alternatively, they were a commoner who was through a season when the orc raiders came through, and the Baron raised a cohort to go deal with them, leading to several weeks of campaigning. They know what they are doing, now, a sergeant in most armies would look him up and down, and nod assent with maybe a bit of a grunt that he's good enough. Not for the elite squad, yet, but he'll get there.
The data is consistent with either picture, so either choice is acceptable.