Every noble was a knight, but not every knight was a noble.
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I could have brought up a number of men-at-arms or some of the members of the White Company that started in a similar position to Rowan were accoladed and enobled but they aren't well known.
As the nobles that were Knights were "True Knights"
Milites Nobiles and the commoners who were mostly mercenaries that known how to battle mounting a horse were "Calvarymen"
Milites Gregarii (making some of them called later as "man-at-arms"), this weren't so common between mercenaries, specially because the horse usually to see in the battlefield wasn't precisely raised by every farmer around all Kingdoms, this kind of horses, now common to us when we see police officers mounted or horse races were specially raised and trained for War, something that wasn't cheap... Think in this mercenaries is like to think in any card game style with characters type SSR and you bought a pack of cards expecting to have at least one, this was more or less when the nobles hired mercenaries, because if he needed to had them as Calvarymen then they had must be trained and equipped in the basics, something that needed time and again, expensive; but even all that consideration, during the Crusades some companies like that existed at serve of Military Orders. The primary reason i discard why Rowan couldn't be a mercenary before the War it's because of his youth during Act I in Seeds of Chaos, i imagine a Rowan who came from a farm at a very young age, recruited as soldier knowing how a bow work properly from his passed Father, wanting revenge for the lost of his family and home by the Demon Armies, the Rowan showed to us looks too young to be a man with so exceptional skills and mastery of the war.
Similar no... Exact please, it must came from the commoners and must be situated during Feudalism, because the examples you gave doesn't help me to get out of my "misconception", it isn't a complete misconception the impossibility to change from a Social State (more considering if the Kingdom lacked from this), even the conditions in a Kingdom that had that tradition was hard to achieve and needed to considerations and patronage of other nobleman, consider that not all Kingdoms in Europe had the Ennoblement as tradition (as you mentioned The White Company now called by historians Navarrese Company formed by mercenaries in the late 14th century in the Kingdom of Navarre, which in fact this Kingdom or at least in Spain had it) and only during the Feudalism decadence in some Kingdoms (like France) it was possible for rich merchants families to buy Patents of Nobility. Nonetheless, the Mastery in the War told in game of Rowan doesn't sound properly from a person who came from the commoners, even the White Company i mentioned had as a leader Louis of Navarre, who was precisely a nobleman. So even if we talk of people like Juan de Urtubia who was a mercenary that commands a group of men under Peter IV of Aragon, a man considered man-at-arms, a properly trained as Calvaryman, being chosen to lead men into an expedition to recover the Kingdom of Albany and later lead an army to fought and conquered Thebes and Boeotia, he was taken as Royal Squire of The King and was more closely to learn from the nobles the mastery in the art of war before going to any campaign (and more importantly a mercenary before the recruitment to form The Navarrese Company in the first place), something like that unfortunately isn't mentioned in Rowan's Backstory because it seems that any form of Ennoblement isnt existed as Rowan himself told during the quest in which we must choose for The Twin Feudalism bullshit or Might Makes Right, at least a commoner like Juan had the chance, but Rowan?.
PS: One thing i almost forgot...
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I could have brought up a number of men-at-arms or some of the members of the White Company that started in a similar position to Rowan were accoladed and enobled but they aren't well known.
Being Accoladed isn't the same as being Ennobled. An Accoladed while is considered as the same as "Praised" or "Award" or "Honorable" as a Knight, they still aren't nobles. To be a noble need more to be Accoladed by recognition of your work and services to the Kingdom, need to be given a Title and the acceptance of the King and The Court, for his merits, his manners corresponds to those in the nobility and the support of at least one nobleman.